The apps must be coded to accept the fullscreen key and then tell GTK to make the window fullscreen. Direct ports of desktop apps usually don't support fullscreen or other hildon features.

I just added this feature to ASUI but it is a special feature that doesn't require an update.

Run apt-get install wmctrl, might need qole's repo.

Create a new command button in asui-settings:
Name: Toggle;Full;Screen
Action: sleep 1;/usr/bin/wmctrl -r ":ACTIVE:" -b "toggle,fullscreen"
Root: [ ]
Unmap: [X]
Double tap: [X]

Enjoy!

Btw, I have fixed the issue with ASUI crashing when reloading its config but haven't tested and uploaded it yet. You will need to use asui-settings Services to restart ASUI if it locks up after adding the button.

Edit: The apps will resize to fullscreen or back to window mode but they don't internally know the new state. Apps that support fullscreen button will appear to ignore it after you use wmctrl, pressing it again will work properly. The app just needs to re-sync its internal state.