In Bridgy backfeed documentation it says "Bridgy detects and sends webmentions for … GitHub comments and emoji reactions on your issues and pull requests" which would seem to imply that Bridgy does not yet send webmentions for emoji reactions (reacjis) on POSSEd comments on GitHub issues and pull requests, e.g. the reacjis on this comment POSSEd by Bridgy.
According to GitHub API docs: List reactions for an issue comment it looks like this information is available in the GitHub API. It would be great if Bridgy could add support for backfeeding reacjis on POSSEd comments! Thanks for your consideration.