There is also the counterpart: 'untag-of' posts (https://indieweb.org/untag) which are more of a brainstorm at this point, but I figured worth pointing out as a future enhancement, in case that helps generalize the code for 'tag-of' support.
Problem: currently you have to use Github’s web UI to remove a label from an issue.
If you could post an 'untag-of' response to a GitHub issue, and Bridgy Publish recognized that, it could call the GitHub API to (attempt to) remove the respective label from that issue.
The current brainstormed markup is similar to that for a 'tag-of' post, but with class name 'untag-of' instead.
Feedback and contributions welcome on https://indieweb.org/untag
Also reasonable to consider 'untag-of' support a potential future separate issue rather than as part of implementing 'tag-of' support.