It would be great if someone tried this out. I can't help thinking though, that a spin that satisfies the mid-girth rule, even if you could furl it, is not going to roll up into a neat package.
Scarecrow hit upon the solution earlier, have a look at
http://www.facnor.com/uk/products/asym_spi_furlers/default.asp which sort of explains it a little.
Nothing very fancy just use a standard furling drum with D12 as the furling line and note the very neat mod of putting a short line to the centre luff edge of the sail, it pulls the centre of the sail in before the ends, it just a matter of adjusting that length to get the smallest tightest roll on the D12.
