I wanted to follow up on this. As of version 1.5.0 (released today), this is now possible natively in Mousetrap.
For your specific example above all you have to do is
var element = document.getElementById("some-element");
Mousetrap(element).bind('enter', function(event) { CallSomeAjaxMethod(); });
You can also create a new instance of mousetrap to bind multiple things.
var mousetrap = new Mousetrap(element);
mousetrap.bind('space', _handleSpace);
mousetrap.bind('enter', _handleEnter);
Best of all it should be backwards compatible so all your existing code will still work.