Total success. First drive just blew me away, I can't wait to go on track...
Installation time was about 20 min or so, it's all extremely easy. I used another poster's instructions for the outer bolts (thanks bigjae1976!) and encountered absolutely no problem at all.
1. No need for seat removal, bolt comes right out
2, 3. Bending the tab to avoid contact with the seat bolster.
4. Rear bracket install. A note here: the Schroth-supplied bolt was too big (diameter) so I reused the OEM bolt. Schroth supplied a spacer which might be needed to clear the MINI seats, but was totally unnecessary in the E90.
5. Minimal peeking from the back seat.
6. Straps connected to the back.
7. Fits perfectly and there's plenty of adjustment left in the straps.
Needless to say, Schroth puts a label on the harness warning you that you might die if you install this in something other than a MINI R55, 56 or 57 so if you decide to follow my example, it's all on you.
Over on my side, I'm deliriously happy with the result. I'll post more pics and impressions on how it holds up after my track event Mon/Tue at VIR.