Incidentally, I became interested in battle models last night, and performed some simple file swaps, to observe behavior.
If I swap Sephiroth's battle model with Cloud's, I get a perfectly working Sephiroth battle model, with Cloud's moves, and full control (as in, no AI data is contained within the LZS file, apparently).
If I swap any of Vincent's limit break battle models with Cloud's, I get a perfectly working Chaos/DeathGigas/etc battle mode, with Cloud's moves, and full control (which is pretty cool).
If I swap any enemy model with Cloud's, the game will freeze as the battle begins, just before it pans down far enough for me to see what Cloud looks like. Apparently, enemy models are different enough to cause a crash.
I decompressed CLOUD.LZS and ENEMY016.LZS and took a look around with a hex editor, doing some comparisons, but I haven't nailed down anything meaningful yet. The wiki contains nothing on the subject. Has anyone done any work in this regard?