On the page about traveling with children, UA only says this:
Strollers may be checked to your final destination without a fee, in addition to your normal baggage allowance.
No mention of the child needing to be with you or not, so kind of YMMV.
The baggage page doesn't really list exceptions for car seats, but says this on the baggage FAQs asking if there is a fee when checking a stroller or car seat:
No. When traveling with a child, you may check one stroller and one car seat free of charge.
That suggests you need to be traveling with a child. United also answers a question about wheelchairs like this:
United accepts one wheelchair or other assisstive device per customer in addition to the checked baggage allowance at no additional charge.
Not sure if a car seat is considered an 'assistive device', especially if you are not traveling with the child, though one agent put it that way a while back when I was confirming I could check both without an issue.
IMO, this all leads the answer to be YMMV. Though my guess is most agents will be able to waive the fee if you simply explain you are meeting the child at your destination.