receiver hitch idea for carrying bike...

After 4.5 years of suffering with a trunk rack for my mountain bike, I'm
ready to take drastic measures to get a hitch rack on there so I can access
my trunk with the bike on the car.

My problem is a regular receiver hitch won't work... My driveway slope is
too much, so when I go in and out of the driveway as it is I scrape the nose
and the tailpipe (car's a 2000 model). A receiver hitch is lower and
further back than the tail pipe and I'd never get in and out of the driveway
with one, and certainly not with a rack installed in it.

So the drastic measure here is to actually cut a hole in the bumper cover
and allow a 1-1/4" receiver to protrude from the bumper skin. The idea here
is that it'd be high enough off the ground to allow me to install a bike
rack and still get in and out of my driveway. The car's not brand new so
the pain of knocking a square hole in the bumper cover is not so stinging.

My current plan is to buy a 1-1/4" receiver box tube from a trailer fab
outfit, and just U-bolt it to the underside of the aluminum bumper on the
rear of the car using square U-bolts and nylock nuts. I have not measured
but it looks like to get it to come flush with the exterior of the bumper
skin it'd have to go about 7" or so behind the bumper.

The question is, do you think the bumper will support the load of a 30-lb
mountain bike and a 20-ish lb rack hanging on the bumper like this?

FWIW I got this idea looking at the mini-fini rack on a mini... Which I
considered retrofitting to a Miata but it's very expensive and I already
have a receiver hitch rack I use with my other car.

Any comments?

I forgot to mention, obviously, this is for my Miata. 2000 model.

C'mon someone here who knows a lot more than me about the structure of the
bumper attachment to the chassis can offer an opinion on whether this is a
terrible idea... ??