Personally, I don't know how it could ever not have lag. I assume the bluetooth headset must buffer the audio to some extent. How is it supposed to then sync the audio to the video which is being decoded by a completely different device/program?

I don't think you'll be able to get the AV in sync without making some adjustments to affect the video delay. What I wonder is if there's any type of existing signaling standard for players that may be A2DP savvy to handle this automagically.

I'm also not sure how much delay you're talking about, but we (humans) can generally perceive audio that's off by even a small fraction of a second. I'm talking like 100 milliseconds in some cases.