I got a Walk In Interim EAD issued from the Arlington, VA last week. They only do EAD's on Wednesdays. My advice would be to go in at about 1:30 pm. The process is a bit haphazard - someone comes into the room and hands out handwritten tickets/numbers around 2. The arrival time really doesn't seem to matter - folks standing closer to the door got lower numbers the day I went. However, long story short - they call in appointments at 2:00 and 2:30. After they are done, they call in the Walk Ins based on the number issued. Per my understanding, if you get a ticket, they will process your iEAD (unless you have an RFE on pending applications).
The funny thing is that they don't seem to want to advertise that they do walk in EAD's. If you ask them, all they say is that you can 'try'...