Is it practical to glue stuff to tiles with hot glue?
I would use a two part epoxy for a permanent install.  The only way you're getting it off is to chip away the underlying tile.  

Construction adhesive such as PL400 would also work.  It's used to glue concrete pavers together when building a wall. However, it can become brittle over time and break off.

If your attachments are ceramic as well, then the proper way to do it is to chip out a tile, and install the attachment using the same tile adhesive used to install the tiles.  Usually you can buy things like a soap dish sized properly for such an installation.