This might be a good question for the remodeling forum, maybe? I see you've been waiting for an answer for awhile on this forum. Maybe someone over there will know something. I know there is a tile guru that frequents the the bathroom forum... Bill Vincent is his name, I think.

good luck!

I didn't respond to this, because you thought 1/4" would look odd. How much thinner than 1/4" would you be willing to live with?

I was also thinking of doing this, and was planning to use 1/4" backerboard or 1/4" sheetrock. But then, like you, I worried what I would do at the edges where I would have to find a way to hide that 1/4". I was thinking of using a capped tile, but then decided to forget the whole idea.

Possibly, you could use 1/8" masonite or 5 mm luan plywood. I don't honestly know if either of those substrates is suitable for setting tile on. You would have to make sure it was well-fastened to the wall so it could not flex and delaminate your tiles.

Ditra might work. It's an orange poly material distributed by Schluter. My contractor used it when my mosaic panel and crema marfil tiles were different depths. FWIW, it's also a water barrier but it does provide structure for movement. And, not sure that it wouldn't also damage drywall when removed!