Tbh, the best way you can train for pump in your arms is by spending lots of time on steep ground (whether the fun way of doing steep routes, or the shit way of going round and round a little panel of climbing wall). The same applies here - spend a lot of time on steep slabs, whether by going up and down a steep slab outdoors umpteen times (which you have the time to do you idle cunt), or travving round a steep slab indoors on small footholds. If you were really keen you could build a footboard, but that might be a bit much.