AutoHotkey -

This could be another role for AutoHotkey (free), if the current Fitaly doesn't do the Windows keys. You'd make Fitaly produce an alternative keystroke - it doesn't matter which - and you'd set AutoHotkey to watch for that keystroke and substitute the Windows key.

Do you use it in combination with other keys? I think that's within AutoHotkey's repertoire and probably could be accommodated with Fitaly - you'd have one action for key pressed and another for key released, perhaps, or the AutoHotkey script that you'd write would take the next key pressed as well, whatever that might be - more or less - and then exceute the combined keystroke.

Address of AHk below (it's what you'd expect) - there's a new version every week or so, I think the fellow just likes to fiddle with it.