Working on designing a new assisted living facility, and I have finally convinced the owner to go with 2x6 walls with 2" of exterior XPS on 1/2" sheathing and cellulose insulation.  However what is the best/easiest detail to build the window openings to allow solid nailing for the fin fiberglass windows.  My first thought was to use 2x8 to frame out the opening, butt the 2" foam into the 2x8 (would stick past the edge of the 2x8 about 1 1/4") and then just cover the extra 1 1/4" thickness with 1" foam sheathing then screw the window through that into the 2x8.  This would be about 1/4" shy of flush, but I think that might actually work well as it would give the fine something nice to recess into thus allowing it to be taped off better to the foam face.

Any other thoughts or ways of doing this?

Build plywood bucks out of 3/4" ply and secure as normal into that or attach brick ties or whatnot to the windows and attack inboard at the wall stud level... Combine the two for a bombproof installation. Minimize the thermal bridging if you can.

Just curious - you can't put the sheathing (only) on the outside of the foam and glue and screw the whole wall together?

You could, but it would be redundant sheathing, as well as being stuck out on the cold side of the insulation. Simplest is to just install window as an 'outie', with the same technique one would use to secure windows when placed in a brick / stud wall.

I placed 2x frames around the RO when I did mine, only to have my window installers pull them, rip them down since they mismeasured wall depth, and reinstall below flush with the foam... Talk about a pain in the ass. 3/4" plywood buck, and you're golden.if you don't trust the plywood to hold, stick a 2x under it for more support.

I think I will actually bump up to 3" of XPS (2 layers of 1.5).  I think I will stick with 2x6 framing around the RO, then use 3/4" bucks as you describe.  But I think I will add solid furring around the entire window as well and screw the 3/4" to the furring, flush with the exterior.