I believe that the transom brackets just screw into the transom, I don't think they have backing plates and nuts/bolts, I think they are a sheet metal style screw, but I'm not sure.
Either way, on the Outback if I do have to get to inside of the transom to get to nuts & bolts it shouldn't be too hard, the back floor comes up really easily and a few 12" extensions would get me in the right place, if need be.
2013 Outback V