I think flirting in a fun way in T can help add some lightness to the session, as long as there's an understanding of the intention behind it and the expectations of it.

My T suggested that he bring in treadmills so that we can do therapy and exercise at the same time, to promote better health. I told him that he would need to wear speedos...and that if it's going to be that kind of work, at least I could get some enjoyment out of it.

Not sure if that would be considered flirting, but we both got a good laugh out of it.