The only place to look for an authoritative interpretation of FRC rules is the official FIRST Q&A forum. The pertinent section of that forum for 2008 can be found
here.
The attachment method described by the OP would comply with <R08> as written and officially interpreted on the forum linked above; however, the bracket(s) would have to be attached to the standard bumper using strong stiff (threaded) fasteners, and be removed from the bumpers when they are weighed during inspection. The weight of the bracket(s) would count as part of robot weight, not bumper weight. See
this Q&A response, among others.
__________________
Richard Wallace
Mentor since 2011 for FRC 3620 Average Joes (St. Joseph, Michigan)
Mentor 2002-10 for FRC 931 Perpetual Chaos (St. Louis, Missouri)
since 2003
I believe in intuition and inspiration. Imagination is more important than knowledge. For knowledge is limited, whereas imagination embraces the entire world, stimulating progress, giving birth to evolution. It is, strictly speaking, a real factor in scientific research.
(Cosmic Religion : With Other Opinions and Aphorisms (1931) by Albert Einstein, p. 97)