I'm more of a trout fishing but in my explorations I come across good spots for smallies quite often. If I were going to fish the Casselman for bass I would most certainly focus my efforts around Markelton. That's all I really have to day about that. You'll find good numbers of fish there.
Cassel is just now down to a really good fishing level by the way. It's been really high most of the Spring. I like it to be 200-400CFS at Markelton on the USGS site for best fishing. Much then that and it is really carrying a head to steam towards the Yough.
Grey, I live very close to these two points. Between both points you will find pocket hole with good numbers of bass and the occasional trout. If you can go downstream of Fort Hill bridge there are a few cold water streams that flow into the Casselman. These spots will produce well. Let us know how you do.