Wow, that turned out to be simple. Though at some point we should probably replace these integer flags with proper enums. Probably post-1.4.0, just in case the change breaks something.

Discussion question: in both Cilph's and Peppie23's version of the code, the flight computer will stay in CUSTOM mode (mMode = 7) when you click an attitude button. This basically amounts to the GRD+/- buttons having no effect until you manually change to a different mode. Is this desired behavior?