Yep, just mount them above the frame so they're in the zone.
In 2010, the bumper zone placement (and an infuriating combination of rules regarding frame perimeter) meant we couldn't mount the bumpers to the sides of the drivetrain. You can't see it in
this photo, but we took lengths of AM C-Base and mounted them about 1/4" out from the drivetrain, then mounted bumpers to that.
This is also why you design your robot to be an inch or so below the limit all around.