Hey pbox,
Look for the resource with type 0x81ca1a10, this is a table with cutout info, and it defines the mappings. For the Base Game there is one single resource that contains all the cutout info for all objects. For the packs (EP01, GP01) they are loose (i.e. one resource per object) which is actually a bug that we may fix that at some point (it's basically an optimization to load less files).
The trick with what you are trying to do is that if you want to replace just one cutout entry in the combined resource the only way I can guarantee it will work (without more investigation) is to override the entire resource which of course is brittle since you are replacing a lot of Maxis-authored data unnecessarily.
Otherwise, you can try to override just one resource that is also defined inside a combined resource. I am not sure it will work though as it will depend on the load order as to which one will win. Note that load order might not be deterministic, although I suspect it is. You might get lucky and it just works, but I can't say for sure off the top of my head.
The table format itself looks fairly straightforward, but I am sure it would help if I provided a BT file. Let me know if you need that and I'll see what I can do.
-SGMS
