Well, I tried it out, and it seems to work. (I have only tried popping
up a Tix file selection dialog so far.) I had to recompile Tix with
Guile's versions of Tcl and Tk, though, otherwise I got core dumps.
The Tcl auto-loading stuff caused problems until I realized what was
happening. Adding a call (auto_load 'tixExFileSelectDialog) solved it.
I think a more elegant way would be to enhance defined-tcl-command in
Gtcl.scm to also call the Tcl auto_load procedure and let it try to
resolve the name.