IMHO I would not use the bumper for anything mounted. I have seen many a time bikes mounted to the rear bumper only to be dragging down the highway of which it is far too late before the owner noticed something was wrong. They are weak, and as stated above would crumple under any "bump".
If you want to mount bikes and do not have the bike rack option go with one of these frame mount setups.
Then you can plug in a tray or good bike rack without any worry. Another option is the front receiver which I have on my TV and it works great for many things.
You also may still be able to order the tray system for your unit as another option. Even then there are disclaimers in the manual that the tray is "only for bikes". CYA litigation at every turn!