The purpose of your abs is not really to make your upper body curl up, but to help you stand upright, or keep your midriff rigid.
When standing with your abs tight, instead of the load just going through your spine, it spreads around through your abs. When in the pushup position, keeping your body straight - rather than arse in the air, or peeling off the ground like a band-aid - requires the use of your abs.
Thus, prone braces will work. But any exercise can be, is and should be a core exercise. Standing upright with good posture requires the use of abs, thus standing upright with good posture under load requires more use of abs.
Squats, lunges, deadlifts, overhead presses, all these basic compound exercises will work your abs... and other muscles, too.