The toy engine doesn't use Tk at all. I have a pretty good idea how to wrap the whole thing in a Tk GUI adding some bells and whistles for the reader/player. Also, this could be expanded to provide a model IDE for games written for the toy engine. Build a GUI for world building so the user could draw maps and insert objects graphically. Tk could be made to do such things with a little effort.
What bothers me about playing around with this toy engine is that the games themselves would be written in Tcl. It would be nice for the engine to parser Inform or TADS files and act as a sort of compile-less interpreter.