The special prim could be scripted to allow general or specific avatar(s) to "sit" on it and the prim could include scripted positions and animations for either or both avatars, to animate both the wearer and the sitter. For example, imagine 2 avatars walking together under single control- without needing to be allowed to rez anything!
I realize that this concept may involve a combination of additions in scripting, Avatar configuration, and possibly additional categories, as well.
If both such attached avatars could also TP together, all the better.