Looks like I get smacked for "shooting from the lip." Apparently I was repeating something I'd heard, rather than actual code. Of course, you must forgive me, as I've never seen wiring in ducts (other than a short length to power something that was part of the duct system!).
300.21 and .22 seem to be the primary place where the NEC discusses wiring in ducts, with numerous other sections that boil down to "OK to run plenum rated wires in ducts." There ARE some other qualifications, so check it out.
