I have the exact same Problem. It seems that the on screen keyboard can't be viewed ontop of the fullscreen application. Here is a post on Stackoverflow with the same Problem and some more explanations (probably posted by you?):
http://stackoverflow.com/questions/24426425/windows-8-tablet-does-not-display-tabtip-on-screen-keyboard-on-top-of-qt-openg

Did you find any solution?