What would be the best frosting for using on a frozen buttercream transfer?  I really have no idea, but would like to try this technique for a decoration.

I always use Richard Snyder's flower buttercream recipe for transfers

1 pound white shortening, 2 pounds powdered sugar.  Mix with white Karo syrup to the desired consistency.  Keeps a good month in the fridge.  Works best if the shortening contains trans fat--this recipe is ONLY for piping separate items NOT for covering or borders.

This will crust/air dry if the items are laid onto paper towels.

Most any crusting b'cream recipe will work just fine.