[mott]

My understanding of the various bumper rules and Q&A responses would lead me to believe (not official obviously) that notching the rigid bumper backing is NOT allowed. If you can't install 8" of unaltered bumpers on each "corner" of your frame perimeter, you'd likely be illegal.

If your appendage attachment points are within 8" of the corners of your frame, I'd say you're going to need to alter your hinge points to be legal.