[Nuttyman54]

The first part of R21 reads:

BUMPERS must be constructed as follows (see Figure 4-8):

A. be backed by ¾ in. (nominal) thick by 5 in. (± ½ in) tall plywood or solid, robust wood. Small clearance pockets and/or access holes in the plywood backing are permitted, as long as they do not significantly affect the structural integrity of the BUMPER.

Emphasis mine. You should be good to go, as long as they're small pockets and don't comprise a large portion of the bumper backing.