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. [Linked Image]

Greg Fretwell