Maybe saddle pad covers? I've seen those before..can't remember where. Basically it was a fabric bag for saddle pads (western) with a zipper at one end so show pads could be used more often or just regular pads could be used without getting super filthy and more customizable.
I'd buy one if they weren't too much. Probably not to ride in, but my pad is microsuede..which looks really gross with dust in it all the time. Right now I cover it with my saddle blanket that I don't ride in when it's on the rack so it doesn't get all dusty and gross. It'd be cooler than a cheap plastic bag because it could have patterns on it and whatnot.