I fitted a Towsure brakcet to my Lag 2, no problem concerning reversing sensors and its the fixed flange type. Had a bit of a faff routing the cable through to the inside of the car though as there didn't seem to be a grommit anywhere near by and was reluctant to drill the car; in the end i poked the cable through the vent flap on the near side and got to the cables there.
I fitted a Bosal (Think thats the name?) detachable type to my Laguna. When I'm driving solo the and the towing arm is attached it has no affect on the reversing sensors. However, as you should expect, the bleeper will have a busy time when you select reverse with the trailer or caravan attached!!
My only issue is that the 12N & 12S sockets appear to hang low on the part hidden tow bracketry but it's never grounded even when towing.