When I took a college ceramics class, I made small (2"x3") ceramic wedges for supporting work. They were fired over and over again without a problem. They never moved, or had anything strike them during a firing, so I don't know how tough they were.
If you really want something in ceramic, you can get kiln shelf supports/spacers at a pottery supply shop. They also have ceramic kiln shelfs, which are fairly tough. You can score and break them. I've seen people use kiln shelf supports at the opening of the forge to rest work on. They are about 2"x2" and come in a myriad of lengths.
You might have problems with bathroom tile, unless it's unglazed. Even then, I don't know how durable it will be.
Other than that, I'll let the experts on this board give you the skinny on what you should really use.
Good luck.
Brook