As yoklov said, I had to have a small script[1] run a quick check on every page you visit to check if the URL is a Pinpointer URL. As far as I can tell there's no way around this (and I'd say breaking backwards compatibility with HTTP is a bit drastic for a little browser extension like this).

[1] https://github.com/Rumperuu/Pinpointer/blob/master/content_s...