I wouldn’t. Check out the Bambino Mio potty training pants. It’ll buy you a bit of time but not aid her to regress further. I think nappies will be confusing for her.
I would but over her outfit so if she has an accident it will still feel like an accident but protect the car seat. My daughter wore a nappy over her outfit in the car seat for the first few weeks and we explained it was for the car seat only, she still has underwear on and will feel wet if she has a wee. It didn’t affect our potty training experience
I use puppy pads on the car seat for long journeys but always make sure there are places to stop along the way
I’d do a nappy personally! In case she does wet in the car she will be unhappy and uncomfortable - we use nappies occasionally and little girl always tells us she needs one even when wearing one ☺️