I feel like when I was researching RipSkirts I saw mention of wearing them over yoga pants and bike shorts - the #2 length. Maybe go to the RipSkirt Hawaii website, I think that's where I saw it.
I think the fabric might be a little slippery on a bike seat, though. Are you thinking about it for casual riding, where you also would get off to have a coffee or ice cream cone in a public place, or for serious mileage? I'd go for a true cycling item for the latter.