Yeah! Thats how the reddit extension works across different subreddits that have used the links.

You could also list the comments by upvotes across instances. With the instance name by the username. So you can see all comments from all instances and communities

It doesnt need to support the google api.

Gab (the nazi fediverse platform) had an extension like this a couple years ago https://github.com/gab-ai-inc/gab-dissenter-extension/

https://www.engadget.com/2019-03-01-gab-dissenter-comments-extension.html

The plugin could work across any website so YouTube and PeerTube would both be supported