Electrical outlet above HVAC air return, within same wall cavity
The air space is just that if the wires are within the air space in any way they need to be plenum rated power wiring even low voltage requires the plenum rating.
the only way around this Is to enclose the wiring or isolate from the environmental air source OR use plenum rated cables.
In your case if I understand it correctly the vent would be above the receptacle but in the same air space no isolation.
A electrical fire in this case could spread more easily without plenum rated cables and the toxicity, burn rate of standard cables vs plenum rated is the reason air spaces need to be isolated or plenum cable’s used.