We wrestled with the same thing this past January, and will now go through it again in a couple weeks. We arrived around 11:45, and parked at the small exterior lot, not checking in through the main window. Then I walked into the terminal to get my boarding confirmation and select our dining time. At that time, there was no choice to eat in the dining car, and I'm afraid it might be the same way this coming January also. Once I checked us in, we drove to a nearby shopping center, and had lunch in our car. Then we drove back to the terminal around 1:45, and we sat on the benched outside of our sleeper car until boarding began, so as to avoid crowds indoors. (It was cold, but bearable. Let's hope the mild weather holds out for us this time around).
FYI - I was just checking the Train Status page, and AutoTrain delays have been worsening day by day for the past several days, and the 12/24-Christmas Eve Train is now showing as CANCELLED !
That's got to be horrible for anyone booked on that day.