Well, it now appears I was completely wrong about this idea being unfeasable, as today I more or less got the code to do what I wanted it to, albiet with a few nasty hiccups I am going to have to address.
So I woke up this morning with the sudden idea that, perhaps, instead of trying to make this work simply by adding religions to the game, I could make this work by creating triggers that would force the game to do it anyways. For those who aren't code monkies, triggers are segments of code that fire when certain things happen, in this case when a new religion is founded. Turns out, adding powerful triggers is actually pretty easy in civilization 4, as most of the event code is written in a scripting language called Python.
That is to say, it is easier than writing the engine from scratch or making a new game, but it is still fiddling with source code in a real, industrial strength programming language (in this case a scripting language), and in this instance one I have never written any code in.
Thankfully, the Python syntax was easy to figure out given my familiarity with other, similar languages. The problems I encountered today where not so much a difficulty with the language as they were with the complete lack of documentation on the part of the developers. See, while they made all the code easy to modify, they neglected to write any easy-to-browse documentation for it, and while I found out later it is possible to glean some understanding of what functions are available from the headers of the C++ source code (I don't actually know C++, I just know how to read headers), the only real documentation I could find was a user made api site, which hasn't been updated since the last expansion pack ><
So this caused some problems. First, I think I accidentally screwed up a ')' somewhere, because the event code for the whole game got completely messe dup and I had to start from scratch after 2 hours of mulling over it. Then one of the functions I heavily needed to rely on required arguments the documentation did not mention and I ahd to guess what values belonged in it, which could possibly come back to haunt me later, then I had to search the code to find classes of data that weren't mentioned in the api etc.
But I got it working, sort of. I managed to alter the game such that when Buddhism was founded, it would be immediatley unfounded and Taoism would be founded instead. This may sound like a dirty hack, but it is basically what i want to happen in the mod, because the religions that are going to be excluded do need to be founded to prevent them from being re-founded later, and it might make an interesting game mechanic to have unwanted religions drifting about. The only problem left, however, is that while the mechanics are fine in what I am doing (I think), the player and his opponents are still notified that Buddhism has been founded, even though there is no sign of it in the game. I can't figure out where in the code these notifications are sent to the player, so I'm not sure how to handle it. I posted a thread on a civ4 modding forum about it, and I'm going to give myself a rest from modding until tomorrow. Hopefully I can figure out how to finish this off, and then all I have to do is test it thoroughly and then start adding religions and identical triggers to my heart's content. That part will take time, but this is by far the most difficult part of the mod to do, and I seem to be a good part of the way done, though it is still possible the messages cannot be blocked or that I might have to change the way I am doing things to stop them.
It is also unfortunate that, presently, my mod isn't well written for integration with other mods. I know what I need to do to remedy that, but don't yet know how to do it. But that is secondary to getting this to run. The alterations I'm making are small enough that enhancing integration should not be too painful.