I emailed one of my POIs at Maryland to ask about the open house decisions since I'm trying to piece together a trip to the east coast to visit other schools where I've been accepted and he said that their open house invites have all been sent out, but the not getting an open house invite isn't a solid rejection.
I'm assuming what happens is that in the event that either things don't go well for some people at the open house or people who attend the open house accept offers of admission elsewhere, other people who they interviewed could, in theory, be given offers or at least put on the waitlist. To me, it seems like a long shot that that would happen, but it's a possibility.