I was considering that Indy. I'm going to try using a derailleur on this one to try to get at least 3 speeds on it. If it doesn't work, I may just adapt it with the worn out brake pad technique I used in the Winter Build OffLooking good, nice stretched ride.
Not trying to play arm chair quarterback, just offering a suggestion. Dept store BMX (mongoose) use large rear dropout plates, you could cut a slot in the red tube vertically and slip that plate in. Good weld surface, would give you the look of looptail bmx frame, and would give axle adjustability for chain length.
Not sure if youre planning single or multi speed. The above suggestion loses the derailleur hanger.