For my last trip out of IAD, I ended up parking in Rosslyn and taking the 5A as I didn't want to risk Herndon-Monroe. But I took a drive through the park and ride a few days ago when I was in Reston and can confirm that overnight parking still seems to be OK at the garage. Signage is the same as it was a few years ago, saying only "no skateboarding, commuters and authorized persons only"...nothing about overnight parking.
Until or unless this changes, still a very well kept secret and IMHO far preferable to the IAD long term lots. I'm flying out of IAD again on the 11th so I'll give Herndon-Monroe a try.