Overriding Bangle.on() not working?

Is there a proper convention for names of objects created by boot code, or am I too paranoid about naming conflicts?

Well, you could just "search all files" for your variable name, and if it doesn't turn up you should be fine.

But do you really need a global variable? If possible it's cleaner to wrap stuff in a function scope:

(function(){
        const origOn = Bangle.on;
        Bangle.on = function(event, listener) {
              // do stuff
              origOn.call(Bangle, event, listener);
        };
})();