Weird, I thought I posted this question a few days ago.
But has anyone ever tried a skillful approach? I mean with around 200+ in bluff you should be able to lie your way through hell, and with 149+ in diplomacy everything you meet will be your fanatic follower. And if you have +119 in survival, you can find Dispaters throneroom by triangulation.
I mean it's not sure fire, but getting to those stats, need not take more than the item familiar feat and some skill points, which with the given stats is hardly a cost at all.
I'm sorry, but no matter how persuasive you are, you aren't going to find out where exactly it is within the Iron Tower. Dispater is probably the most paranoid person in the Planes (certainly in the Nine Hells), and other than his highest ranking courtiers like Merorem, Glasya, or Titivilus, every other devil, Duke, and Archdevil would have zero information. Hell, Merorem, Glasya, and Titivilus probably don't even know for sure. Paranoid as Dispater is, maybe he's made them think they know where it is, but kept the truth of its location hidden. And even if they did know for sure, it's not like you'll be able to get the answer from any of them, not with the knowledge that the second they told you, Dispater would have them obliterated. Talk about a negative circumstance bonus.
That said, I'm sure you could ask Asmodeus. Good luck getting an answer that doesn't result in eternal torment though.
Remember that if I get my diplomacy check to hit the 160s consistently (so a +159 to the roll is needed) (which should be possible with 70HD and a DR of 18). I can talk any of his associates, or even Asmodeus into being my fanatic follower in a full-round action. And I could get those as immediate actions with celerity, meaning I would just have to have a way of getting loads of immediates to make anything that I want my fanatic follower who would impale himself on a spear if it helped my cause.
As such, I think that they would tell me all they know about the throne room as well if they have any doubt to whether it is the true throne room.
Even so, I don't need their help to find the throne room. I just need them to help me not being found by Dispater (which I should off course ensure with a bunch of spells). I can find it with Survival, a DC 120 survival check finds me the direction of a place I've heard the description of, well I should be able to glean from CoP that dispater sits on an Iron throne in an Iron Throne room, and thus be able to find the direction of that with a survival check. So I'll just find the direction, teleport in any direction that is not that direction, find it again, and draw meself a map (with a sufficient Profession (cartographer) and survival skill off course). Draw in the directions, and teleport to myself where they cross.
Since his fortress is truly infinite, they will only cross in one place.
Even if this will not yield a result, I could just make a almost impossible Bardic knowledge check to find it out, which means I'd just have to stick some levels in bard and get an insane Int score (such as abusing tattoed monk and consumptive field).
Also since Asmodeus knows where Dispaters throne room is, if I go to his planar home first (which is easier), and mind rape him I can get the information I need and then teleport directly to the throne room?
Heck, I could even just go there and tell all his minions to obey me, in their
dreams (scroll down, the rules for lucid dreaming are there).