A-ha. Universal has 28-32 with a 48mm schrader stem - the 9th one down. Can probably stuff 'em in 25mm tires if you wanna, just make sure you don't have tube under the bead. I wouldn't put 'em in 23s personally. Doubt you can get Schraders that skinny.

Universal Cycles -- Q Standard Tube Cyclecross Tubes