Tip: learn to use grep. It is a wonderful tool.
casino:~/Documents/tremulous964 ben$ grep -RHn "ClientCommand" src | grep -v svn
This command finds all teh instances of ClientCommand in src and its subfolders, prints them with filename and line number, then removes any entries that contain svn (because I'm not interested in the .svn folders).
It came up with quite a lot, but only one actual call to ClientCommand:
src/game/g_main.c:310: ClientCommand( arg0 );
A closer inspection shows this as a part of the vmMain function, which suggests it gets called from outside the game.qvm (I think, although this is not my area of expertise)
It might be worth now tracking GAME_CLIENT_COMMAND or possibly looking at the SV_ClientCommand function in src/server/sv_client.c
I'm not sure how relevant those two are, but I'm not going to spend all day finding out for you, either.