There are some MTB shifters that are not integrated with the brakes levers, the real trick with Northroads bars is finding an orientation that's not too cumbersome to use. I think thumb shifters are going to be your best bet.

Stylisticly speaking, grip shifters aren't going to win you a date with the hipster of your choice, but they actually work quite well and are unobtrusive. I liked the set I had on this Univega and they were easy to shift despite the funky swept bars: