you might get a cruising speed of 40, but everywhere else, the ride-ability will suffer.
might work good at bonneville, though...
you'll probably need a flip flop hub, but as kat pointed out, the lockring will be going in the wrong direction and would probably thread itself off.
for a lightweight rider ( i'm 135) and a hopped up motor, 28 is about the lowest you can go and still be able to ride comfortably at any speed.
the track cog, flip flop, and left side drive hub ideas have been kicked around the forum for a while now, but no one's been able to make it work, as far as i know.