1-Nov-24 UA1348 IAD-RDU A320 (N446UA): Asked Captain after boarding, he knew exactly what I was talking about, and explained it wasn't available on that particular frame yet, but would be in the future. No doubt he would have turned it on if it were available.
1-Nov-24 UA689 RDU-IAD NEXTified A319 (N834UA): Asked Captain after boarding, he also knew exactly what I was talking about, and flipped the switch. Listened gate to gate. What a joy! 👍
Thank you LarryJ for the suggestion to visit the flight deck and ask the Captain in person. Success!
I already sent in kudos for both Captains via the Feedback route.