Rohloff's Downhill tensioner
— looks quite rugged?
are routinely used for cyclocross, downhill, etc
there's LOTS of variations on these.....
lots of praise for these
pictures of how they're set up on a DH bike
MOTORCYCLES regularly use chain-guides
my biggest concern is making sure the drivetrain can handle enough power/torque... you're not going to tell me that motorcycle drivetrains can't handle a ton of power/torque