Get at it from underneath. I had both bumpers off my E91 to get them painted. Front one not too bad. Rear one pain in the ass. As anyone who has removed the rear bumper from an E9X will tell you there's a series of cut outs on the bumper that go onto the bumper brackets. These are below the rear quarter and most of the time on removal these tend to break as you will see on any for sale on eBay. I'm 99% sure you could do what you need from underneath.
E92 330D LCI
NSR 250 MC21, CBR 600RR 3