FYI -
I was out riding today and met a couple dirt bikers marking trails for Merrimack Valley Trail Riders Cystic Fibrosis benefit ride. Last year's ride was canceled, but apparently this year they are all permitted and ready to go.

I could not find anything on their web site mentioning Bear Brook, but there are signs up in the Biker/Hiker parking lot.
I think Bear Brook is the 'Southern Loop' they refer to.

I think the ride is scheduled for 10 am, riders should be all done by mid afternoon.

The route includes part of the one mile trail, podunk road, and broken boulder trail, other than that I dont know.