Daox, did you ever move the dehumidifier from the basement into the bathroom? I did a google search for something and found an ER thread.
I would think an in-ceiling dehumidifier to replace/work with the fan would be your best option. What's above that ceiling, anything like an attic or crawl space?
As far as an automatic thing, you could put your Arduino skills to work here. They have a device to tell you the moisture content of soil (good for your garden
), you could use that to tell the board to send or not send a signal to a relay connected to a dh/fan. (Maybe they have a better tool, but the idea is there.)
Another thing to consider is a personal pet peeve of mine. I hate getting out of the shower and it is freezing cold in the bathroom. I have to keep the door closed to keep the bathroom at least at a comfortable temp. In your area, this is going to be even more crucial, especially during the winter!
You have to consider what an enclosed shower will do to the rest of the bathroom and what it will do to your comfort level.
Side note, funny story. On your pic of the bathroom, there is an ad attached (sorry, not a super-duper awesome supporter) and it's an LG with lots of clouds that move up. It makes it look like steam is coming from your shower!
