Yep, that is cool. Most non-glazed ceramics will work, but I had never thought about a mug. I use ceramic rods to put the final edge on all of my knives.
Yup, that's the way we have always sharpened our knives at the old hunting shack. The back side of a ceramic tile is also very good. I have found that water works much better than whetstone oil when using ceramic material.