I haven't transferred in this way at IAD, but have never run across restrictions at any other airport. I don't see why OP couldn't use the "recheck" counter (aka transit desk) after Customs, just as if they were transferring between two UA flights. If they want to be sure, they could contact Delta in advance to make sure they have reps there. AF and Delta certainly must have an interline agreement regarding transfer of bags. If they were transferring to Southwest, sure, no interline agreement, so no transfer of bags, but I don't see why AF-to-Delta should be problematic.

Do make sure that if you bought any liquids, cremes, etc. over 3 oz. in Duty Free, including on board the flight, and you have thos ein you carry-on bags, that you move those to your checked luggage before handing those off at the transfer desk or check-in counter, so they don't get confiscated when you go through TSA security on your way to your Delta flight.