Roger, I think you should pursue this. I don't know if you already have parts in mind, but if you look for auto brake drums, you can find them 12" in diameter, and up.
Being able to turn down the rotors isn't the only way to adjust the "air gap" - you could use shims; wrap a steel sheet around either drum to close the gap if necessary. Start out with a wide gap and close it up as necessary. A stainless steel band around the inner ring of magnets would hold them on easily, and allow disassembly to adjust air gap, as already mentioned.
Holding the coils would require a disk with the coils standing up, not embedded in plane with it... not as efficient structurally, but you could do it. Alternatively, you could get a large PVC/ABS drainage tube, and cut holes in it. At a 10-12" diameter, the wall of the tube might be about 1/2", giving you lots of room for wire. There is probably a temperature penalty, so be mindful of the cooling, as others have noticed. Otherwise, your main problem will be weight - this will be heavy compared to an axial flux of equal electrical performance, IMHO.
BTW, cool graphic.
