WJG (17/04/08) Has anyone tried this one out? http://www.gtk-server.org/intro.html
'The GTK-server is a free, open-source project, which offers a stream-oriented interface to the GTK libraries, enabling access to graphical user interfaces for shellscripts and interpreted programming languages using either GTK 1.x or 2.x. It was inspired by Sun's DeskTop KornShell (dtksh) of the Common Desktop Enviroment (CDE) for Unix.'
There's a sample script for tcl here: http://www.gtk-server.org/demo-stdin.tcl.txt
Judging from the examples, the API one gets with GTK-server is at about the same level as one would get by using SWIG on the GTK C headers, i.e., it's a lower level interface than Tk. Might in that respect rather be comparable to ffidl and TWAPI.