I’ve forgotten who sent this link to me, so I apologize for not giving credit. I’ve posted lots of videos and photos of brass players using transparent mouthpieces, here’s Brian Kane playing a transparent tuba.
He’s also playing a transparent mouthpiece, but the camera doesn’t focus on it to see his air stream direction. Probably downstream, but that’s always a probable guess simply because most players are statistically more likely to be downstream.
Kane’s comments on the diameter of the tubing and how it affects the upper register of that instrument is interesting. Building a brass instrument to play well in all registers is a complicated thing.