Very good idea!
I had already thought about integrating something similar into my site (https://moecki.online).
I have not yet programmed a browser extension either. This is a good option for users who trust the extension. But I think it would be better to integrate this signalling into the condenser.
For example, posts that are paid out 100% in SP will show this sign:
Such an icon could also be displayed for posts with a beneficiary to @null.

Figure out why highlighting doesn't get applied when I scroll further down in the feed

With infinite scrolling, the "new" posts are dynamically added to the page via javascript when a certain area of the page is reached. It may be that the extension then has to be triggered again so that the "new" posts are also detected.