I meant to update earlier, but on our trip over Thanksgiving - we were able to check through on the outbound from SFO to IAD to Newark.
Coming back, the guy in Newark couldn't do it and insisted that the connection time (5.5 hours) was too long. So in IAD we had to go from the C/D gates to the main terminal get the backs, recheck them and back to C/D gates. The employee who took the bags on the re check told me that the Newark guy should have been able to do - the actual maximum is 12 hours.
Oh well, it was a bit of a pain to walk to the train, etc but we did have plenty of time.