That's a good question...when my son was an infant and still breastfeeding, we didn't yet have that backseat rule for the car seat - the car seat could be in the front passenger seat but facing backwards and I used to just put the tip of my pinky in his mouth until we got home and I could feed him properly - although sometimes - yeah-I'd just have to pull over and feed him in the car (it seemed he'd always get hungry right as I was driving through some dicey neighborhood in East Philly on my way home from work).
I guess I'd just say, give yourself plenty of time so you can pull over if you have to. You're probably in that habit anyway, as when you have an infant, everything you set out to do gets interrupted in one way or another by one or another of his or her needs.
I'd just make sure s/he'd been fed and changed, try to find some soothing music that s/he likes and play it while driving- make sure they're warm enough but not too warm, etc.
But I would hate that car seat in the back seat facing backward rule.
I'd miss the eye contact and opportunity for interaction myself. Maybe you could put something interesting for the baby to look at on the back and side windows of the car - as well something that'd keep the sun from shining in the baby's face.
I guess if none of that works, you just have to stop the car and hold the child til s/he calms, or if you're sure nothing's wrong and you can't do anything about it, try to get to where you're going as quickly (but safely) as you can.