Focusing on the B concourse at IAD, in addition to the VS and NW lounges noted above, there is also an AF lounge that the ANA site says ANA passengers can use while their lounge is being renovated.

While I doubt that AF will allow LH pax in the door, it would be interesting to test *A limits.

I'll be flying out of IAD on LH in a couple of weeks and will test the B concourse lounges and post results. In the meantime, there are always the UA B and F lounges in the C concourse.