You are certainly on the right track, realizing that it is a violation of code to fish NM down a air duct. In the events of an air return duct such as you describe, that is only made up of the studs, it used to be, and still is, I believe, permissibly to pass across the duct and seal the holes drilled.
Yes, drill a generous hole for the NM and run two lengths of it in that hole, and flare them away one from the other. Short parallel runs are permissible. Use some fire rated expanding foam to fill the hole after you are done.