Why not foot on exercises (on a chair behind the board - at whatever distance back to make it more/less overhanging etc..) and then you can take one hand off/shake out etc.. and then swap hands etc..? This will also work the core as you're compensating for having one hand on/off? Then just keep this up as long as you can!
Heelhook big holds on the board instead of stepping off?
the good one would be to set up those fingery boulder problems that are so unfashionable at my gym
now I will start doing sets of one max dead hang immediately followed by one max pull up, for 15 times in a row. 6 sets of this. clearly I will have to change the holds accordingly to progressive tiredness.
in case anyone is interested. i did my first session of this yesterday. it's heavy. it packs in 90 pull ups and roughly 15 minutes of pure dead hangs. i alternated harder and easier sets according to fatigue but still my forearms nearly exploded! great fun!