I also do them on my forearms and toes. Even though I cannot hold that position for the full 90 seconds required by PBF, the easier variations don't do anything to my core muscles, they just hurt my arms and wrists, especially after the push-ups and pull-ups. BTW I think it's much better to do planks first, before the push-ups and pull-ups and overhead presses, when my arms still have some strength left in them (and before the squats, when my legs still have some strength left in them, lol).
Height: 5'4" (1.62 m)
Starting weight (09/2009): 200 lb (90.6 kg)
No longer overweight (08/2010): 145 lb (65.6 kg)
Current weight (01/2012): 127 lb (57.5 kg)