The HOV lanes are only non-HOV3 after 6pm from the Pentagon to Turkeycock, where they become HOT lanes. You are still required to have 3 persons to ride free in the HOT so drivers still need to pick up slugs if they want to drive for free from that point on.
I haven't had to test the slugline that late yet but my boss does and he says it's still pretty much like before, there are drivers still picking up after 6pm. If anything, there should be more drivers coming through, especially if they are heading south of Springfield and don't want to pay.
I know I would be interested in picking up riders for Stafford after 6pm however I only drive on Friday. As I am in Alexandria then, I'm reluctant to take the time and the gas to drive up to the Pentagon to see if there is anyone there to pick up. I tried posting something but did not get a response from anyone.