I haven't pulled off the bumper yet to have a look at the area under there, but looking at this post: Removing the Rear Bumper [Guide]Well, kind of.
Both radars need to be mounted behind the bumper, on either side.
Some kind of bracket work would need to be made to mount them firmly and keep them positioned correctly (longitudinal axis angle and vertical angle).
5Q0 and 2Q0 radars perform dynamic calibration as the vehicle is being driven so any positioning imperfections (within limits) would be compensated by the software.
The area in the bumper directly below where the sensor would normally go seems to have a similar angled face. If the sensors have some self-calibration like you say then mounting it with the original bracket with maybe a few washers as spacers if necessary could be a feasible option right?
Getting the sensor to be plumb and level shouldn't be that hard, right? If that's the only real hurdle I'm thinking I may attempt this challenge.
Unless I'm missing something of course....... Though I've only just bought this vehicle and it's got less than 700kms on it so far. I'm also in two minds about whether I'm ready to be drilling holes into it...... ha ha ha