Voiding the warranty and carrying a couple of bikes is two different things. My Cruiser Aire 5'er has a bumper with a factory 2" hitch coupling. My understanding is it will hold 300 pounds. I intend to plug in a rack and carry two bikes (certainly less than 150 pounds, probably closer to 100.)
I think Crossroads says "no warranty" because they don't want to be holding the bag if someone drills the bumper and supports full of holes and/or applies a bunch of backyard welds to haul a motorcycle or golf cart off the ground. However, the actual square bumper is pretty thin so I would be leery of even a clamp on bike rack on it due to torque caused by the racks height.
Bottom line IMHO is first determine if it is feasible to safely mount the bike rack. If not, move on. If it is feasible, then decide if you want to chance the warranty question. Of course if you can safely haul bikes, the warranty question shouldn't come up. If you have to extensively modify the bumper or frame it is probably not a good idea to begin with, both safety and costwise.
A final thought: If you do mount something, be sure it is rigid and doesn't bounce around creating additional stress on the bumper/frame. Just my 2 cents.