If you still cannot secure E+ prior check-in, politely mention to the GA that you'd like the chance to move into E+ as any UDU's clear. Most likely, the pax that get the UDU at the gate will be in E+.

I've done this in the past and it's been hit or miss, since you're going SFO-IAD, the E+ would be a major benefit so it'd be worth you hanging near the podium so as not to get out of sight of the GA. My experience has been if they see you and you've been polite they remember to help you out. Last time I tried this I was moved from E- into F. It was probably more due to my status and fare clss, but I'd like to think being patient and friendly helped out.