Another possibility is to use a quick release seat post rack on the mezzo, a little bit more fiddly and they usually have a limit of 10kg.
YES, BUt not sure about over stressing seatpostclamp.
Any one else had problems with the seat post clamps cracking. I have repleced 2. Mezzo ATb should add a hole drilled in them to stop the crack going further than where it tends to oringinate. (Near the clamp rear edge)
However i often run a caribina with bags attached hanging from the seat post rails.