I've been a fan of this combination for a while, but I've only just reused it to explain the shenanigans.
What do you mean prepare the divine spell in the arcane slot... spell versatility doesn't affect preparation - it explicitly states you prepare spells separately and normally. OH I understand what you are getting at for archivist / wizard, that's a totally different story though.
There's nothing, anywhere, preventing you from preparing spells you can't actually cast in your existing spell slots. You just can't cast them, which would make it a waste. Wizards/Wu Jen/Archivist all draw their spells from spell books, which by the wording;
She must choose and prepare her spells ahead of time by getting a good night’s sleep and spending 1 hour studying her spellbook. While studying, the wizard decides which spells to prepare
Means that when she selects her spells from her spellbook (Blessed book of Boccob in this instance for ease of having the spells), she can select any for the slots, provided they are prepared by a spellbook. So; Wu Jen 3/Archivist 3/Geomancer 10 (all Wu Jen advancement)/Incantatrix 3/Archmage 1 is my favourite example. It has 9th level Wu Jen Spells, and 2nd level Archivist spells. Regardless of spell level, you can transcribe it within the spellbook.
To prepare the spell, you must be have an Int of 10+Spell level, nothing else. The only time that there's a "check" to ensure it's on your spell list is when you cast it. The definition of parameter is extremely loose. However, a parameter is a "limit". We're talking about exploits here, so permissive ruleset for CharOp. In my eyes, being a particular type of spell (Arcane/Divine) and on a particular spell-list are both parameters. As such - both are eligible to be changed AT THE TIME OF CASTING. So that "Miracle" you transcribed in your Blessed book using Archivist that you prepared in your Wu Jen 9th level spell slot as a Divine spell from the Cleric spell-list you cannot actually cast, until, AT THE TIME OF CASTING, you change its parameters to being an Arcane spell on the Wu Jen spell list.
It's an exploit, and, from what I can see, RAW legal.
This is regarding a thread on GitP asking to break the Geomancer.
Anything that anyone can spot that's now RAW legal?
Just a few notes as to why I like it. Ban Enchantment, because those have the least useful spells. Have Archmage use its SLA to get Miracle 2/day. You can now Persist (because why else would you get Incantatrix?) every spell in the game. Add on any other metamagic shenanigans - I'm thinking Reach here - even though you might "touch yourself" with a spell, you can now reach it, and now persist it.
The main trick is by using Extended Persisted Reached spells applied, casted either by my own spellcasting, or by the SLA Miracles, which I effectively have unlimited of, due to Body Outside Body (SLA's, unlike spellcasting, are not explicitly banned by BoB), a 7th level spell which is possible to be cast via SLA Miracle, QED, Ever exponential, ad-infinitum.
As an aside; at Epic Spellcasting, all these unused Epic Spellslots can only go towards one thing - burning a spell slot during a Ritual Cast (use the Healing Seed for free Instantaneous).