Some random brainstorming: the highest per-round damage in the game comes from the charging combinations, and mounted charging has an interesting synergy with smiting because the multipliers you get from wielding a lance, Spirited Charge, etc. multiply your smite damage so even without Power Attack you can achieve some high damage numbers. I don't know if there's a way to make an actual build out of this, but I'll throw out some ideas and the links between them. The big problem with mounted combat is that if you're Medium, you need a Large mount that can't always fit inside wherever the party might be exploring.
The two premier smiting classes are ordained champion, which you found, and fist of Raziel (BE). There are some other classes that advance preexisting smites like the Destruction domain's, but none, as far as I can recall, that advance spellcasting and a previous smite at the same time. Exorcist of the silver flame is also a worthy addition to either. As far as mounted combat goes, the feats you want are Mounted Combat, Ride-By Attack, and Spirited Charge. For charging, you can probably do without pounce, not that it wouldn't help to create a truly awe-inspiring smite nova, but you still might want the standard charging feats, Power Attack and Shock Trooper (CW) with its prerequisite Improved Bull Rush, to add even more damage. You can take Leap Attack (CAd) but it's only useful when you're not mounted so it would be a backup feat for when your main schtick doesn't work. The two classes that give good bonus feats and advance divine spellcasting are ordained champion, again, but you have to swap out domain granted powers which you may not want to do, and hospitaler (CD), which conveniently has Mounted Combat and Ride-By-Attack as prerequisites. One other class you might want to take a look at is prestige paladin (UA/SRD), specifically because of the spell
rhino's rush (SC 176-177) if your DM would allow you to take it, which adds another charge multiplier. The valorous weapon property (UE) also adds another charge multiplier. For a different approach, the White Raven discipline (TB) gets a stance that adds to charging damage and several maneuvers that also add fixed damage on a charge that you can probably multiply with all other stacked multipliers, with ruby knight vindicator advancing both White Raven maneuvers and initiator level and divine casting, as well as providing some very useful abilities especially since it's quite possible you'll end up with a swift-action overload---ordained champion's smite's big disadvantage is that it takes a swift action, meaning you can't nova it and can't use it while casting a swift-action spell at the same time. Finally, if you're planning on investing a lot into your Cha, which is not impractical because of your insane starting stats, marshal has an aura that adds Cha to charging damage. I don't know much about Eberron deities, domains, etc. so it will be up to you and your DM to see if any of these things can work together. Technically, RKV is Wee Jas, ordained champion is Hextor or Heironeous, fist of Raziel is Raziel, and so on.
The best mounted weapon for this is clearly a lance because of the multipliers. You can also use it one-handed while mounted if you're not using Power Attack for sword-and-board since your Str bonus is pretty negligible on top of all the other modifiers you have. Other than that, there's an argument to be made that going with something with a good threat range (18-20) with a
scabbard of keen edges will give you another multiplier some of the time, especially if you have fist of Raziel's auto-confirm-on-a-smite-ability. Alternately, you can get some fairly ridiculous damage by combining a smite with a weapon with a high multiplier, x3 or better x4, and
surge of fortune (CC 128-129) to force a natural 20. IMO, there are very good reasons for using a reach weapon unless for some reason you can't, and since none of the other reach weapons bring much to the table for you, I'd just use the same lance mounted and unmounted unless you're looking at criticals. If you use the same weapon, you can also just stick merciful on it once if you want less violence as an option.
As for getting a worthy mount, the best choice is always Leadership if your DM will allow it. Beyond that, you can always purchase mounts. This can be a fairly reasonable alternative at lower levels, but at higher levels of purchaseable mounts only the steelwing (MM5 168-169) can really survive melee, and it's Huge. (Clerics of appropriate alignment can animate serviceable mounts but this isn't an option for your character.) IIRC, there are some magebred mounts and the like in Eberron that could work pretty well. The
summon monster spells can get you temporary, largely-disposable mounts starting at about III. With a cooperative DM, the
planar ally spells can be good ways of obtaining mounts that can survive combat conditions. Prestige paladin will get you access to the special mount feature without hosing your character, but unfortunately it's hard to progress special mount without taking more class levels and there aren't many good feats to boost it without investing class levels. The Mount Handbook,
http://www.minmaxboards.com/index.php?topic=25.0, has more ideas.
I am not at all impressed by Dragonmark Smite, Smiting Power, Silver Smite, Improved Smiting, Exalted Smite, or basically any of the other smiting feats you have access to except for Awesome Smite. (Other smiting feats can be good, for instance Devoted Inquisitor which takes sneak attack.) I recommend Awesome Smite because it handles two banes of weapon-users at the same time, DR and concealment. From an optimization perspective, I don't see much that a Dragonmark is doing for you, but I'm not familiar with all the Eberron material so there may be something I'm missing. Mark of Recovery and Sentinel Stance in particular are terrible feats. As for your other feat choices, I approve of Practiced Spellcaster and Holy Warrior but note that your build as-is is not legal because Persistent Spell requires Extend Spell, and you can't swap the Law domain's granted power for both Law Devotion and Power Attack (or at least, there's no way I would allow that). You definitely want Extend Spell anyway at some point for 1 hour/level spells like GMW and
magic vestment. You may want Quicken or Persistent Spell, though you're approaching swift-action overload with the former and the latter doesn't come into play until quite late without Divine Metamagic abuse. If you want to break the game, Divine Metamagic is always a strong option for that. Craft Wondrous and Craft Magic Arms and Armor are solid choices if your DM lets you use them to break WBL or won't let you pick arbitrary magic items without them. I question Knowledge Devotion: while cloistered cleric has the Knowledges in-class and the skill points you need to buy them all, the rest of your class levels are probably going to be 2 + Int skills with more restricted Knowledge access. You're effectively spending two feats (Able Learner and Knowledge Devotion) plus a ton of skill points for that bonus to damage, and I doubt it's worthwhile. To me, the Knowledge domain's power looks like a prime candidate for swapping via ordained champion, and then just skip Knowledge Devotion. I'm not even convinced that cloistered cleric is worth it for you since BAB is vaguely important for prerequisites. Law Devotion is a good power for all weapon-users; Protection and Good Devotion can be useful under some circumstances but I'm not convinced you need either of them. None of the Silver Flame domain powers are particularly useful to you, though you may want to look at the spells still.
