I just took a quick look at keyproxy's source code. It uses X Window calls to do the magic of pushing a keypress event into wine. It certainly won't work on a standalone Windows computer.
An idea for those of you who can write Windows code:
http://www.google.com.sg/search?q=simulate+key+press
Write a "keyproxy"-like program that listens on the network for triggers from FG, then simulates a PTT keypress (you may have to inject this keypress into AVC directly).