Just to echo the responses so far. If you have a valid PP on you, you can access the lounge. It's unfortunate that the RCC is the only PP lounge at IAD though, as it's really only marginally better than sitting in any of the airport bars.

I recently used my PP to acess the GOL Smiles landside lounge at GRU while waiting on an intercity bus that stopped there. They asked for my boarding pass, I showed them my bus ticket, they made a little notation on the PP log and handed it back to me. I was then free to help myself to those addictive cheesy bread rolls they serve.