If the soundbar supports AirPlay, you could try casting the audio via AirPlay to several devices simultaneously - to the soundbar and to the headphones. It works with AppleTV and multiple AirPods for sure, as I've used it before.
It's a somewhat stupid idea since you're sending audio data to the soundbar (which is connected by a cable to the TV) via wireless radio (so the cable is essentially useless), but it should work.
If the soundbar supports AirPlay, you could try casting the audio via AirPlay to several devices simultaneously - to the soundbar and to the headphones. It works with AppleTV and multiple AirPods for sure, as I've used it before.
It's a somewhat stupid idea since you're sending audio data to the soundbar (which is connected by a cable to the TV) via wireless radio (so the cable is essentially useless), but it should work.