Considering that there are 5 different types of car seats, and they can be adjusted to about 10 different configurations that's not going to be an easy task.

The people who design car seats these days know that their customers would rather spend an hour rethreading/reinstalling their car seat every six months than spend over $200 on the seat. But if there were a large fleet of shared cars on the road, and if some entrepreneur came out with a $1000 car seat that could be readjusted in seconds for a child of any size, then the owners of the fleet would have an incentive to loan out those seats along with the cars.