Rowan, am I missing something? I don't see how the attachment at the rear dropout is going to work. 1) what prevents the extension just rotating up, ie the back end folding with the dropout sagging to the road? Don't you need a brace from the extension "seatstay" to the bike's seat cluster to keep that from happening? (Even if the bolt through the dropouts was tightened that securely, I would expect it to twist or break. Plus, I would expect the bolt to bend as it is considerably smaller effective diameter than an axle. (For bending, the threads don't count except as crack starters.)
I'm not trying to get down on your bike. I just feel like I am looking at a suspension bridge with the middle one foot of cable missing and hoping that it will be installed before they open the bridge for traffic.
Ben