I was at my LBS shop yesterday and the mechanic had installed a 42T cog on the shop owners Scott 710 MTB. He was having trouble with the top jockey wheel hitting the 42T cog when shifting into that gear. He tightened down the B-screw all the way to provide as much clearance as possible but it the chain would still rub the 42T cog if the clutch on the XT r/d was engaged. The only thing he could do was not engage the clutch, that provided like 1mm of clearance so the chain would not rub.
However, the r/d did shift onto the 42T cog easily, just not enough clearance once the r/d moved under. The only fix I could suggest was to check into the One-up r/d cage although the top jockey wheel would not change even doing that so you would need to somehow move the whole r/d away from the cassette by a few millimeters which would mean a different r/d hanger. In the end I think they ended up putting the old 36T cog back on the cassette.
This is probably a case where if the rider wants a Shimano 1x setup with a 42T cog, he will need to upgrade to this new Shimano XT 1x11 offering.