frameteam- That's a first I've seen. Very neat! Yours also has a moving derailleur (in addition to chain tension). I was thinking of having a fixed mount chain tensioner (pivot only) mounted to the chainstay by two bolts. An internal sliding dog would be in the hub center, attached to an external 3 speed type cable parts. A 3 or 4 speed cassette cog set would be joined as one but be able to loosely slide on the splined body. The splined body would be partially slotted, allowing an opening for the internal dog.