A gentleman who is considered one of the most experienced tiltrotor pilots in the world having spent decades flying the XV-15, V22, and 609 had this to say about slope landings in tiltrotors:
Slope landings were done on the XV numerous times with the preferred direction being upslope using nacelle angle to match the slope. We did these up to 15 degrees upslope in mild wind conditions and kept some thrust on the rotor as the brakes weren't the best. Downslope we used 95 degree aft nacelle limit plus some aft cyclic which translates to a bit extra flapping and landed on 10 degree slopes.
The max we demonstrated sideslope was a bit over 5 degrees due to potential nacelle strike issues with crosswinds.The preferred method is upslope with a tiltrotor as you have much more capability with forward nacelles (dial in nacelle angle to match the slope)but should the situation require other directions , these can be accommodated within the potential limits mentioned. If you recall, we even hovered at 60 degrees nacelle (30 degrees nose up) just to demonstrate capability but no qualified aviator would attempt a slope landing at that absurd angle. Great air show maneuver however, and has potential for target tracking in an attack configuration.
This all should apply for the V-22 AND 609, but can't speak for the current flight manual limits, and the 609 has not demonstrated capability as of present time."