No problem with that. But if you're using flexible duct, make sure you don't provide low spots that will collect inevitable condensation over time in extended seasonal off periods. And go for a rust resistant diffuser. There are extruded aluminum or painted aluminum registers available. They won't rust, and they look alot more substantial as well.

Thanks!

janwad,
Reflecting on what I told you a day or two ago - while I was standing in front of my electric heater this a.m. after taking shower .... Not sure what you're trying to accomplish with the vent in the shower ceiling. If it is the expectation of a warm stream of air blowing on you during or shortly after a shower I'll caution you, when you're wet, that stream of air will not FEEL warm. If it is however the best location available, I'd say o.k. Hope I'm not too late.

Oh dear, that's an excellent point! We have a 2 stage furnace that blows a lot and I've noticed that without even being wet!

I bought a home 2 years ago that has a vent in the ceiling of the big master shower. It is freezing whether it is winter or summer. I put a new vent cover that is supposed to aim the air away and I keep it closed (but it is never 100% closed). I am trying to either figure out how to move the vent or to relocate my shower. I know this was 11 years ago, but I hope you decided not to do it because I think you would regret it.