I know of a local CMA club (not the WC place I train at) that does the following exercises :-

1) Plunging ones hands into a bucket of rice, and grabbing a handful of rice. Let go of the rice when you withdraw the hand. Repeated for minutes on end as fast as possible. Purportedly strengthens fingers.

2) Finger and thumb pushups.

3) Conditioning forearms by hitting them against a WC wooden dummy.

I have no idea whether or not these exercises work, or whether they result in injury, because I haven't tried them.