I know that Rip addressed wrist flexibility for the front squat in SS 2nd ed., probably in 3rd too. He outlined a way to stretch them with the bar.
When I was getting my wrists accustomed to front squats, I would put my palms flat on a wall and slide them downwards gradually. I think you can do this with your fingers facing up or down to get a slightly different stretch (slide the hands up the wall if fingers are pointed down).
You could try wall push-ups with your wrists at various angles until you can eventually do a regular push-up.