Adding messages to alarm.json?

Could there be a way for an app to create an alarm that when triggered could load (execute) that app itself to display the alarm

Yes - the README on the WIP branch for it is here, but it'll do exactly that: https://github.com/espruino/BangleApps/tree/new_alarm/apps/sched

Right now there common code, but no common UI because, honestly, the alarm UI isn't great as-is so I imagine it'll want changing soon anyway :)