I see no problem with it at all.. Say someone has a broken leg and he wants to play in his specially designed wheel chair.. Or better yet someone who doesn't have legs but is gifted enough to be able to hit the golf ball from his chair..
penalizing them would be unfair, unless the ruling bodies feel that they are getting an advantage?
I said "in reading greens."
A 3' level would likely span too much of an area and the slope would likely change a little. As I said, one end or the middle might be elevated slightly because of a valley or ridge. Greens aren't perfectly planar. A 9" level is a good compromise: big enough to get an accurate read, small enough not to span bumps and valleys.
Take an AimPoint class.