I love my EB/MM JPM too, but it hasn't driven me to bust a nut yet.
First loosen the strings enough to where you can get a clean shot at the nut. Then, using a toothpick, fill the crack in with some super glue (after masking off the fingerboard & headstock with tape of course!). Then, apply pressure to the nut and squeeze it together. Hold it for a couple of minutes at least so that the glue can set up. Wipe away any excess glue. As long as there aren't any chunks missing from the nut, this should work fine. But, be sure to give the glue plenty of time to set up and harden before you tune the guitar back up (at least overnight).
Otherwise, you could probably get a replacement nut from Ernie Ball/Musicman, but I'm not sure that it would come pre-slotted. Even if it was, you'd most likely need to fine-tune it a bit. You could also pick up a Tusq or bone blank nut, but you'd have to slot it yourself, which can be time consuming and a bit of a pain.