I just spoke to Cindy Enstrom at the BLM, she was able to confirm that the land that the trails are on are not affected by the closure. Good news.
Sounds like the posting of public notice was a formality that was requested by the Water Bureau, to clarify that it is illegal to enter on the Bull Run Watershed land. The BLM didn't have a formal notification, and signage is still sparse, so sounds like the Water Bureau just wanted to make it clear that you can't ride in the watershed.
So, when you bike there, and ride the paved road up to get to the Hide & Seek trailhead, if you go further up the road, to the North, you will eventually reach the boundary of the watershed. This was clearly marked w/ a sign the last time I went all the way up there. Cindy (BLM) also stated that there used to be an upper gate to prevent further access via the road, and that there were plans to re-add a gate up top.
Looking at this map provides a visual idea of where the watershed starts/borders the trail system land (towards the top of the map):
Sandy Ridge Trail System
So, in summary, nothing's changed, the trails are still open. They're a lot of fun if you haven't checked them out.