I take back the part about only working for the 2 mouse buttons. It's possible to work around that limitation of the IS GUI features. The workaround isn't as elegant as repeater regions, but will still solve the problems with FTL, etc. To make it work for other buttons only requires adding a Mouseover for the Click Bar button, to toggle some Key Map or Mapped Key(s). I tested it just now with mouse4.

The toggle mapped key goes like this. No Hotkey, set to "pressed OR released", with 2 steps. Step 1 has a Key Map State Action to turn on a Key Map (one that is not assigned to your Character Set). Step 2 has the same thing to turn it off.

In the newly-toggled Key Map goes any mappings you want to add to that click bar button, be it mouse buttons, keyboard buttons, etc. If you want to move your mouse over the click bar and hit G to heal, that will work fine.