Have you tried BFR training? Might allow you to do the pick-ups at a low enough intensity that it would be viable?
You can also just strap the cuffs on and open and close your hands, minute on minute off, until the pain and pins and needles gets overwhelming... I've done this when injured. I'm sure it's far less effective than proper/normal training but also is likely to be much better than nothing!
Not finger flexors, but wrist curls and reverse wrist curls with some kind of PE protocol might also be viable to get fit wrist flexors..