I wouldn't assume that people are holding out.
It's very possible that no one on this board fishes that section.
If you Google "lower Clarion River fish populations" you will find a pdf document from the state with info on studies done on that section of the river.
In a brief look, it seemed to be that the pH levels are on the low side, from mine drainage, and that game fish populations are low also. They found some suckers and chubs, and a few smallmouth, but it looks like VERY few.
But, things can change, and they survey at certain spots, not the whole river. If the fishing pressure is low there, you might find some big old fish.
Regarding access, I found info on River Trail maps for the upper and middle Clarion, and that website said they don't have a River Trail map for the lower Clarion because of very limited public access.
But, the maps show places where the road is alongside the river, and some bridge crossings, so there may be "casual access" at some of those places.
It sounds like a good exploration trip. Go for it.