Two updates:

1) I drove out to IAD today to drop off a few bags of toys and groceries. The pantry location isn't super-obvious to find: Enter on the arrivals-level entrance, walk left over to bag claim 6, then look to your left for a small sign by the outside wall. Follow the arrow on it to walk down a narrow hall and turn right through a door, and you'll find this room. FYI, parking in the terminal lot is down to $6/hour or $10/day, and you should have your choice of spots.

2) TSA has also opened up a pantry at DCA for food and toiletries. The press release doesn't specifically mention donations from the public, but the described location in Terminal A across from the lost-and-found office puts it landside.