No ideas here other than to roll your own. I have not exhaustively searched the 24" market but from what I recall they are generally front and rear geared. I also I have not seen anything other than mountain bike or road bikes in that rim size. There could be some other offerings out there, but I just have not seen them. The restriction of no front derailleur with disc brake may be your limiting factor. You could probably find one with a disc and front der., then remove all but the outter chain ring.