We did something very similar in 2010. I don't have any close up videos of the mechanism itself, but it was almost exactly the same, and we didn't have any problems with holding strength, and we had more then a few very hard hits. As for the "bumper support rule" (<R07> K), it was our understanding during inspection that because the bumper was in contact with the frame that when the bumper was hit it transferred the load directly to the frame, (ie it wasn't hanging off the side causing the bumper to bend) and therefore was supported. However our robot was quite small last year and our mounts were never more then 8" apart anyway. Have a look at the diagram in the section 4 <R07>-K, the right most scenario should answer your question.
This was our change out time
http://www.youtube.com/watch?v=PQRya...layer_embedded
and this was how we made them
http://www.youtube.com/watch?v=PcZjm19kcsg&NR=1
http://www.youtube.com/watch?v=BaHGN...eature=related