Ok this came to me in a dream. And, oh yes, this is my first post in the frame builders forum. Has this ever been done. If not there is probably good reason why it has not. My illustration is terrible, but it shows dropouts or ends that are inside the seat and chainstays. Obvious reasons this is a terrible idea are that the axle would have to be removed for wheel removal and spacing for freewheels and cassettes would be totally marginalized. Look at my crude drawing, thoughts?
I don't know if that has been done before or not. Most everything gets done at some point. Since the axle has to be removed anyway, the forward end of the slot might as well be closed for extra strength. In that format it is the way a lot of Chopper frames are done. Maybe not literally, because I can't quite tell where the tubes begin and end in the drawing, and whether that semi circle on the rear of yours is the drop, the continuous tube, or something else.
Cinelli and others had a system where the axle didn't extend into the dropouts. There were fixed cups on the dropouts, and the wheel was inserted. This was shown in "Cycling" which is often called the bible for racers.
I'm not sure there is a huge problem with an axle that isn't fixed, other than that it isn't comon. That is the way it is done on lots of wheels with cartridge bearings. The big problem is that the bearing needs to be a press fit which is inconveninet for roadside rebuilds, and I guess Phil has a way around that. But if there aren't cones then it really isn't an advantage to have an axle with weak threaded ends for any greater depth than required.