Joe- the issue that seems to have come up is the wording "passing through" versus "terminating in" the CA. If it passes through the CA perpendicular to the long dimension I don't see a problem but when you start installing boxes it's debatable. I'm curious how this forum feels about it.
I think the code prohibits it. I also think that it is a silly prohibition. The IRC allows anything, other than gas piping, to be installed in a plenum. For example, I could run a 4" ABS lengthwise through a plenum, and that wouldn't matter. I can't do this with 14-2 NM cable, however.
The "passing through" language is meaningful. When you install a box in a return plenum, the box becomes a return air register. It is just like the dumb idea of PCs "breathing" through the diskette drive. They end up packed with dirt. In a dwelling that dirt could be combustible.
BTW a Florida guy would ask why you are "returning" cold air? That is the supply side.