You'd think that there would be a work-around, but right now I don't see it.
In fact, I think you will almost have to do a little stage-play (assuming the timing of your PHL-DTW flight permits):
Check in
and board together first, smiling at GAs and FAs and making it clear that although the F seat is in your name, of course your wife is going to use it
...
Get her settled in F and move toward the back, when -- gosh, you can hardly believe this, but you just got buzzed / paged for [your excuse goes here] and you need to leave the flight. And as you sigh, and apologize, and gather your stuff, you pull the GA (and/or FA to the side) and say, "This is bad enough as is -- you'll leave her up front, right?" (Plan B: You say nothing and hope that your earlier uxorialness elicits the proper reaction from the other players).
Then you dash off to the DFW gate (which I don't think needs to feature in the boarding and de-boarding drama).
Life's short. If no one comes up with a better solution, give it a try.