Alan, when you read 300.22(C)(2) Exception it tells us that "This section (meaning Section 300.22(C)) does not apply to the stud spaces of dwelling units.
I've had this discussion with Jeff Sargent from NFPA and that's his understanding also. Job is approvable.

For the sake of the disbelievers, I've seen contractors take a piece of metal panning material 3½ by 24" and nail it on above the box and band it around the box and nail it below the box. Now you've divorced the wiring from the air flow.

Very interesting. I'm going back to 'most' of my jobs and do it the easy way!!!

Learned something new today. Tanx!

The exception is for wires passing perpendicular you are still allowed to run horizontal.

Rewire. I was looking at the same place you were, but I think we have different images of what the metal box is that was going to be used. You are thinking, solid box suitable for a dust enviroment I pictured a standard metal device box with screw holes, etc. Could be we are both right or both wrong.
I would have to see it to judge it. Guess that is why inspectors still make house calls.

I used a 1900 box with a 4x4 block of wood covering the back ran emt horizontaly from the adjoing stud void this was the fix I cleared prior with the local AHJ.

I see what you're trying to do now...

The last time I did this, I used a putty pad to wrap the box.

Caution, I have my HVAC hat on.
As a design issue I would want the box sealed to the plenum air, just so it does not become part of the return venting system and pack with dirt. George's idea of the metal deflector would work but you are also compromising the effectiveness of that plenum for returning air. If the HVAC guy computed that he needed a 14.5 x 3.5 duct, you have just choked that down to 11 or 12 inches and if it is not a smooth transition the turbulence effect makes it worse than that.
I doubt they really slice things that finely in residential systems but I also think they are likely to err on the small side most of the time and make ducts smaller than they should be. This will only make that worse.
I agree the electrical code may be somewhat ambiguous on this but you could have an HVAC issue.

This sounds like a really good idea....

From the words of Rewire...

"this was the fix I cleared prior with the local AHJ."

You might also want to check with any fire marshal. Where I am from, the fire sub code people do not want any wire in any duct work.

I like the fire putty pad solution a lot.

You've stopped dust intrusion and addressed fire transmission at one stroke.

Sweet.