Maybe this is just me but I'd be very careful about risking any other part of my body just to strengthen my wrists. Push-ups and the plank require near perfect stability and strength of the spine and hips. Any slip out of the posture can stress both while the load-bearing responsibility falls on already-vulnerable wrists. There are myriad ways to use safer resistance trainer that just isolates the wrists. You could start with something safe and work your way up to push-ups, etc., if and when the wrist bones show gains. Again, this is just my thinking and I tend to be risk-averse about things not easily fixed. Slow and steady seems a good mantra for osteoporosis-prevention strategies in my mind at least.