Yeah i've thought about this before back when i first got my DBV. Although i wanted it to be one-way in that there would be a flap or something that wouldnt allow air to be blown down the hosing. I had problems with some of my friends accidentally blowing bowls into the heatercover and it was getting really annoying lol.
But yeah, it would be cool if there was basically a whip that could prevent vapor from flowing out when theres no inhalation. It could be a mouthpiece with some sort of valve/flap built into it, or an in-line piece that you would attach hosing to on both ends. I think it would be best to have it at the beginning of the hosing though so that vapor isnt sitting in the tubing, possibly condensing and building up on the hosing, which isnt usually able to be harvested like it can be from glass and such.