Interesting project. The outbuilding has hot water from the house or from a water heater?
Is this the kind of tub/shower you are considering?

Could you add dry wall/ board and batton or timber boarding to the lower portion. On the ceiling add dry wall or if less expensive plyboard and cover with tin panels.

If you put rustic bathroom in as a search in houzz, lots of inspiration photos.

Most economical for the cinder block may be to use a concrete stain such as ones with copper sulphate to impart aqua tones of blue and green and attach reclaimed barn wood chair rail. Ordinary corrugated tin roofing also makes an excellent ceiling covering.

Breadboard could work as well. They make plastic sheets which look like wood when painted.

Corrugated metal on the walls below the brick. Houzz has lots of photos of rustic bathrooms using this.

Thank you everyone for your suggestions. My first idea was corrugated roofing below red tile, but read it was a bear to saw through and was concerned about safety issues. Then I thought beadboard, but can't find it in a thickness appropriate for nailing over framing. Have not heard of the copper-sulfate concrete stain, but will start reading. Definitely want to do a rustic chair rail. Board and batton and lattice are more great ideas! Clawfoot tub would be great if I can get a deal; if not, I was considering a galvanized trough. Hot water will be supplied by dedicated on-demand heater.

I'm surprised about the corrugated roofing being difficult. My contractors didn't seem to have any problems when they installed my roofing. Have you talked to anyone at home improvement stores?

Should be no problem with a circular saw using an abrasive blade for metal cutting. If you're doing it by yourself it may be helpful to use a 2x4 to lift them up and a second 2x4 to use as a guide when cutting

Here is the article I read which gives a lot of cautions when cutting though galvanized: http://www.garagejournal.com/forum/showthread.php?t=12191

There is a product called B- Bond, which you can mix and apply directly to the block
walls, which gives a stucco look, and waterproofs the walls. You can paint over it, and can scrub it down when needed.