kujhawk,
I have been able to 'direct'set mosaic in tufa. It is a little different than working with a concrete or mortar mix, as often you have perlite/vermiculite/peatmoss that just happens to be at the same spot you are wanting to place your mosaic piece. It may be helpful to paint the back of your mosaic piece with a little bonding agent or white glue then kind of squish it into place, allowing for a bit of mix to kind of surface around it. In an hour to a day you can usually brush off any excess slurry that gets on your mosaic piece.
Happy casting and it is great to mix your work,
Dena