Have just seen a discussion about this on another site. It seems a fairly common practice - and one I admit I did
when DS was tiny and couldn't be settled any other way and XP wouldn't pull over because he was a cock.
But, looking back our of the fog of new-motherhood, sleep deprivedness, baby screaming hormone-induced desperation, it was a totally mad and stupid thing to do. I might as well have taken him out of his seat, if we'd crashed the full weight of my body would have gone right into him, the seat wouldn't have taken it, and he could have been seriously injured.
So I mentioned this on the site thinking that perhaps everyone who did it had been in the same sleep deprived and not quite thinking state that I was and hadn't thought about it rather than actually believed it was safe, but no, apparently it is possible if you just turn to the side and the baby turns their head to the side.
I have NEVER seen a car seat which would allow for this - not one for a few-month-old baby, especially. Surely it's impossible to breastfeed safely in a moving car, full stop.