I think it's the toggle part that's causing me the problem!

I can see that there are input connections on the individual zone HVAC for "Fallback Enable" and "Fallback Disable".

If I connect a QBI button to the HVAC "Fallback Enable" input then I can press the button and... fallback is enabled.

My problem comes in trying to use the same QBI button as a toggle so on the 2nd press I want to connect to "Fallback Disable" ... or somehow have a 'state machine' that inputs to a generic Fallback connection (which would mirror the enabled/disabled state of the state machine). ... if you see what I mean!

Making the QRI LED behave is the easier part - actually I want it to follow the final enabled/disabled state of the zone value driven by a QRI relay... but I have the bit working so far.