Remote Buddy's XBMC Behaviour only sends commands to XBMC, so that 
toggling fullscreen behaves just like it is implemented in XBMC.

You can nonetheless quite easily achieve what you are looking for by 
creating a custom action (Prefs > Mapping > XBMC > Custom actions..), 
that contains these two building blocks:

1) the Action "XBMC > Toggle fullscreen" 
2) an AppleScript, executed only on release, with this code:

tell application "XBMC" to activate

Best regards, 
Felix Schwarz