What about this? (Nearly 10 years old article using MonoDevelop to program a XLib/Athena widget application as a proof of concept).
A quick search revealed that in '90s WAFE (Widget[Athena]Front End), from the University of Vienna, used to be a relatively popular interface; you may grab a ridiculously old binary and venture in the hard task of finding the proper shared objects to make it work on the Linux ABI (nowadays is much easier to have pre- 3.x Linux binaries work through compat_linux(8) than on modern Linux distros themselves). Revelant research paper from the USENIX Winter 1993 Technical Conference.
