A couple of us (myself and Jake) cut back the overgrowth last year. While it helped, it was way too overwhelming for only a couple of people to do and I ended up injuring my hand/arm because of it. We even tried to get Cascades Cycling Club (who are supposed to be in charge of maintaining the trails) to lend a hand, but that ending up being a joke. The only way to do this is to gather a nice group together and tackle it in one day. This did happen a few years ago and was a great success, but that was the last (only?) time it happened.
Jackson has a nice gem in that park with those trails. It seems like the park is not at all interested in anyone maintaining those trails. Not the CCC or MMBA. This is the reason why I only ride there in the spring and fall. It is also why I cannot wait until the trails at Waterloo are complete and ready to ride.