It's my understanding that the typical 4" square steel bumpers are rated for 100 lbs. max. That would be static weight too. Dynamic forces from hitting things like speed bumps and potholes will be much higher and sometimes enough to catapult things into orbit. That's a lot of stress that can lead to failure of welds and attachments. What you add there would be on top of the weight of the spare tire too.
The only way I would do it is by adding welded reinforcements and attachments to the I-beams. For an idea how to do it well, look at a Nash TT. Some TT manufacturers offer racks at the back as an option but there's no way I'd trust one. You can't see things hanging off the rear and won't know if you lost it in transit.
Not a good idea, but lots of people hang bikes, BBQs, ladders, gennies and other stuff off the rear. It's fun watching them bounce way up when they hit speed bumps in CGs at just a few mph... :)