Not a car seat, but I've checked boxes several times in the past with zero problem as long as they're within the dimension/weight limits. Most often a taped-up paper box with training materials and office supplies in it.

One of my favorites was coming back from a day trip to my company's HQ. While there, boss had given me a bottle of bourbon that I obviously wasn't going to take through security. Bubble-wrapped it and stuffed it into a FedEx letter box. The check-in agent was confused as to what I was doing until I explained what was inside.

Arrived back in ATL just fine - the SC agent that SDC'd me even managed to call down to the ramp to get it moved to my new flight.