hey mjkraw,
I live fairly close and drive through all the time. There is a lot of private property on Plum, mostly farms. I'm sure if you ask nicely there are some who would let you access on their property. I have scouted it out from the road a few times but haven't quite made it in the water yet since it's been late in the year. Water does look great though. Typical small stream limestoner. I have a friend who has caught some nice browns within.
Halter can be accessed from the papermill. I have not fished it yet either, but my brother has and told me it is decent.
Where they combine the water gets even bigger, and it seems to run warm because it has had open water every time I have seen it this winter, but haven't been past since the real bad cold has set in.
If you want to explore it and try sometime, let me know, maybe we could meet up and check some spots out. If you get to it before me, let me know how you do. I will post back here once I give it a shot as it warms up with any more information.