My collision engine does allow for a mesh to have a separate triangle list for collision, but it's still 1 physobject per mesh. I pretty much code everything from scratch except DirectX and Windows, so I don't use third party libraries. It's just an Ellipsoid/Polygon intersection test, I don't have Poly-Poly collision yet and when I do it'll probably be convex polygons only. For now, though, the map file has one mesh that's already fairly low poly, and I've been using the same mesh for rendering and collision because of the fact that it's a hammer map exported into my format.
Technically if I were to take your suggestion of using a single mesh for collision and several for rendering, I would just have to give the collision mesh a nonrenderable mesh and set its physics mesh to the one I had been using, then fill the scene with a lot of non-physics objects set up for culling. Still, though, I'll have to figure out what criteria to cull based on-- I mean, I don't have any sort of area portal system and I'd rather not manually separate them in 3dsmax... though technically that is an option. Another idea though-- I did manage to get a W-based texture selector working using a massive switch case, but it only supports up to 16 textures because of the SM3 sampler limit. However, if I were to chunk the map into a bunch of culled pieces, it'd be easy to have less than 16 textures per chunk at least.
So to summarize, I'm thinking I could turn off rendering on my big one-piece non-textured map file, split it into a bunch of non-physical pieces, multitexture each of the pieces separately, then draw the pieces separately using frustum culling. Technically though if I make a few changes to the collision response I don't even need the one-piece physics object and I can just make the chunks physical as well. Do you see any problems with the switch-based per-chunk multitexture idea? Is swapping out a bunch of samplers per pixel shader painfully slow or something? It sounds like it would work to me, but that massive switch case does make me feel a little nautious