Delta's "Checked Baggage" page:
When you arrive at the airport, you can drop off your checked baggage at the nearest baggage drop, marked with red "Baggage Drop" signage, or curbside check-in station (available within 6 hours of departure time). You may also check your bags at the airport kiosk, ticket counter, or curbside.
It's not 100% clear language (*sigh*), but I think that if they'll allow curbside at T-6h they should also be willing to allow it at the ticket/bag drop counter.