What many people call a "plenum" is actually (in the NEC) " "Other space used for environmental air" Take a look at 300.22 in the 2002 NEC Handbook.

NEC definition of plenum- A compartment or chamber to whih one or more air ducts are connected and that forms part of the air distribution system.

Most of the concern is about smoke. I think UL will be your best resource on this matter.

Few things are harder to put up with than the annoyance of a good example.