Munchkin9, an update on my issue:
After fixing every mesh in my mod to have a "30000" flag, I went in to test and experienced a Runtime error within the first half hour of play. This indicates two possibilities. Either my system configuration is somehow incompatible with Warband (though I should note my actual system specs comfortably exceed the recommendd settings), or there's more to the incompatability of a mesh than just the flag setting.
So I played a game on Native. No custom content. I played for half a day, doing everything in my power to crash it. I took castle after castle in an attempt to duplicate the Runtime error (I usually get it the moment I press "lead your men in the assault"). No crash. No Runtime error. Flawless.
It's very possible that had I kept playing, it might've crashed. But considering the frequency of my castle attack crashes prior to playing on Native, and then considering I took the entire Sarranid Sultinate and half the Kingdom of Rhodoks, cities and castles and all, without a single crash on Native, I'm leaning toward a mesh incompatability seperate from just the flag setting.
I've redone my mod and removed all content that was carried over from the original Mount&Blade. I've also removed a lot of Warband content created by the community. Any custom mesh has been removed. The only things I've kept are my own extensive collection of retextures and the content from Better Banners. So far, no crashes, but I still have much testing to do. If it continues to work without a problem, I'll slowly re-add some of the content I removed and hopefully, by this admitedly clunky and tedious process of elimination, I'll find the mesh(es) causing my problems.
1257 would be a much harder job and take more than one person to fix---at least using my method of in-game testing. Better for someone to figure out once and for all what it is with some meshes that's causing Runtime errors so we can just go through all items in OpenBRF and either fix or remove the problems. Then again, whatever it is, it might not even be detectable in OpenBRF and might be something done at the mesh creation level.
Anyway, there's my report. If I have any news from my new "slimmed down" mod (i.e. a crash) I'll note it here, but so far so good.