I would love to see this feature and it would be what would make me buy a pair of Sonos Aces to replace my current headphones.
Use case: I work from home in the same room with my significant other (SO). We both listen to music on our Sonos speakers all day while working. However, at some point my SO starts taking calls for a while. During this time, she still wants the music playing in the room, but I need to have headphones on to block out the sound of the phone calls. So I put on my headphones, pair to Bluetooth, and have to open a second music streaming service to play music in my headphones while leaving the Sonos system playing music for her. This is annoying on a day to day basis.
I would LOVE to be able to just slip on the Aces and pick right up with listening to the same music in the room and not have to get on my phone and start up a second service, etc. Honestly, this is the only use case I can imagine getting the Aces for given I have my Sony MX4s.