Is it metal, plastic or flex duct?

The main concern with wiring in ductwork/plenums is the ability for fire or smoke to rapidly spread through the home or building. Being unused, IMO, the duct is no longer a duct; It has become merely a passageway. I say go ahead and use it. If you are concerned, run the new wiring in PVC or metal conduit, or flexible metallic sheathed cable.

I have two concerns with using abandoned HVAC duct work as a raceway for electric cabling: first, HVAC duct work is not designed for that purpose; if it's metal, it can have seams and sharp edges which could damage insulation on cabling. That danger could be mitigated by the use of metal clad (MC) cable as noted by the poster above. The second concern is with respect to supporting the cabling which would be running vertically in the duct repurposed as a raceway. Section 330.30 of the National Electrical Code (NEC) requires MC cable to be secured every 6 ft. and Section 334.30 requires non-metallic sheathed (NM) cabling to be supported every 4 1/2 ft. and with 12" of a junction box, outlet, cabinet or fitting.

NM or MC cables can be unsupported in concealed spaces when fished through access points at each end. See NEC 330.30(d) and 334.30(b).