I assume you mean while the car is stopped? I wouldn't try to nurse while the car was in motion; if you are in a crash while leaning over the seat, you could interfere with the function of the seat and injure your child. And YOUR safety belt only works while you are seated correctly; you don't want to hurt yourself OR become a projectile.
The Fisher Price Safe Voyage Deluxe is made by Britax and is a lot like a Britax Marathon, minus some of the extra features. It does not tether rearfacing and only harnesses to 55 pounds, but it's as Britax-like as you can get without actually getting a real Britax. Albeebaby has them in stock.