[Al Skierkiewicz]

Eagle (what is your name again?)
The rule you reference refers to robot structure that backs up the bumper. i.e. the part of the frame not shown in your drawing. The design criteria include the backing, knowing full well a 150 LB robot at any speed would be able to break or severely weaken the 3/4" plywood. ( I think Dave Lavery showed a picture of a of the damage from a robot hitting the wall last year.) With robot structural members behind the bumper plywood, the whole system provides what the GDC expects would protect the robot during most legal game play. Using the C channel in your drawing for attaching the bumper is a good way to attach the bumper, but the structure is still part of the rule. In your example, the 1/8" gap caused by the C channel would not compromise the 3/4" plywood as it is compressed against the frame.
In this year's game, contact with other robots on the bump will cause some interaction to occur at angles not encountered during games on a flat field. That being said, bumpers mounted only in the middle with no backing robot frame structure will suffer from these non-parallel contacts. Bumper/robot damage would be the result.