I was running Mavic 823 rims on XTR hubs when I was up to 275lbs. I never had a problem with my Kenda Kenetic 2.2 inch knobs. I would usually run them at around 40-45psi, but when I got lazy and didn't top them up every few days I am sure I was down to the 30's (didn't use sealant). Since I wanted a little more speed I got some Maxxis wormdrives (semi slick) and would pump them up to 50+. Once I pumped to 55psi in a 70 degree air conditioned room and took them out into 95+ heat. After hammering up and down some pretty steep but short climbs I was descending at about 25 and heard a whap,whap,whap just before a very LOUD BOOOOM.
I would strongly suggest not go OVER the Max psi, but I think they do fine at lower. And sounds like that is what you want for the maximum deformation of the tire to grip the roots without worrying about pinch flats.