:
: Who makes 7-string single coil pickups?

Dimarzio. Possibly Seymour Duncan as well, but I’m not sure. I built a guitar a while ago and settled for Seymours in the end, but I remember the DiMarzio catalogue had Steve Vai endorsing a 7-string pick-up. I think it may even by DiMarzios that are fitted to Ibanez’s 7-string range from the factory.