The front is actually the easy part of this to accomplish. You can use a 2 piece crank designed for a direct mount chainring, you'd just need to create a spacer that would interface with the crank arm's spline on one side and the chainring's spline on the other. Using a 73mm crank in a 68mm bike would let you space the chainring 2.5mm away from the crankarm, with the normal gap between the ring and crank there should be more than enough room for a cover to hide it all. The bigger issue is that the rear cog sits inboard of the frame and to cover it inside a frame design would require having the axle nut and cog in each other's location and that's the hurdle I see difficult to overcome.