What I did was use old shards of stained glass, some are even old 70's lamps dissasembled and cut up, and I tumble the glass in a rock tumbler sometimes, to get the sharp edges off,(but this can be done by scraping the glass edges together to get the edges dull) and I use clear silicone adhesive to glue the peices to the shower door, leaving a space for grout. then I grout it just like you would tiles on a backsplash or tub surround. I have even bought stained glass off ebay! And also have smashed up glass plates, bottles to use in this fashion.

About 8 months ago I removed what I believe is exactly as you've described: a metal framed sliding glass door from a cast iron bath tub with tile up the walls. (I then promptly destroyed the tile and tub, but that's irrelevant to this discussion...) There had been some holes drilled in the wall tiles, but the tub itself seemed to be fully intact. There was a lot of caulk everywhere that would need to be scraped off and then cleaned up with some sort of chemical. (I found a caulk remover from one of the big box stores that was fairly cheap, easy to use, and didn't do any damage to any acrylic tub so certainly cast iron and tile could handle it.) I'm guessing you also may end up with some holes in your tile where the door is secured that you may need to patch and/or cover with the new door.

It can be painted as brass shower enclosures are normally a painted or powder coated finish and possibly brass plated (which I do not know if it will accept any paint well). To make the finish look decent the entire enclosure should be taken down and the metal properly prepared for paint before painting. If your looking to do this yourself and use spray paint the finish will never really truly resemble brushed nickel.

olychick, that is a Kohler hydrorail with a Moen rainhead in brushed bronze. (The Kohler rainhead was hundreds more, so I got a Moen on closeout.) I love, love, love the whole setup. So functional, and designed by Kohler primarily as a retrofit to avoid changing out a wall valve, or a lot of drilling. In the end, we did update our valve, but wanted to keep our simple, clean tile because the bath was very modest inside. I hope to put another in our MBR refurb. i think we lost our OP!