Is it generally code compliant to run romex wiring through a cold air return duct?

In many houses the floor joists are used as part of return --- they

Work well as return ducting -- the wiring should not present a problem...

I am old school so I guess there may be city codes -- give them a call

It is a violation to run NM cable along a space used for environmental air. Crossing one perpendicular to it is fine.

What Stanfortyman said! :)