GreenFrog,
I've been down the Chain Drain several times since the area had public access.
The Chain Drain is easily navigable all the way to the rock chasm and cascades. From there, it's maybe a .2 mile bushwhack on river right to rejoin the Drain, with the confluence with the Rock a stones throw beyond. Downstream on the Rock are a few riffles, and maybe a couple beaver dams (it varies from year to year). Another bit and you're on the Cedar, right where Square Rock Rapids dumps out. If you scramble up and over the hill right there, you'll stumble upon Pine Lake.
I've been there and a couple miles farther downstream, but never paddled the last set of rapids to the Hudson. And if you're curious, I have yet to paddle the rapids between Pashley Falls and Elm Island. Someday, someday.
I have some pics I'll post in a few minutes...