The only 'key combinations' I'm aware of are for example pressing+ and - at the same time on things like tempo, transpose, registration bank select, multipad bank select.
But the assignable buttons can do more that just open menus - and in fact there is an option under the 'Overall' category to assign metronome on/off to one of the A-F buttons or a pedal. Have a look at reference manual (v2) p134 onwards