Using spacers on crank spindle instead of bottom bracket when fitting HollowTech MTB crank to road bb
My guess is that thus setup could work. There should be no problems with the bearings, as moving the cranks slightly outboard is equivalent to moving the riders shoes slightly outboard in terms of forces and leverage.

However: are crank spindle spacers available? I guess plain washers with the correct inner diameter could be used. They would have contact only the bearing inner races though.

You;d also have to check what chainline you get with the MTB crank.