I doubt the top of the ductwork (even if it's not insulated) would get hot enough to do damage to the wires . . . the air temp may go as high as ~150"F in the duct . . . but many attics get even hotter . . . and I haven't heard of any body with issues when wires are run it the attic space . . .
I would suggest insulating the ductwork (if it's not) . . . for energy savings . . .
Pete C
ps. plenum rated wire is really about smoke developed when the insulation burns . . . how much smoke and how toxic it is if breathed . . .