The J community seems to be pretty excited about their QT interface. Tk is easy to use, but results in horrible-looking applications that doesn't integrate well with the rest of the interface.
If I were to implement support for a GUI framework, it'd be either GTK+ or Android, depending on whether I wanted to target desktops or mobile devices.
That said, I have no intention to do either so my opinions on this matters approximately this much: ⍬
Do you have plans to implement this Tk support? If so, I will applaud your efforts and my preferences for other frameworks will not stop me from helping if I can.
Regards,
Elias