We'll have to see if this gets abused. I wonder if folks will sign up as "ringers" for repos that they want to sabotage? That could be awkward. I do know that kind of spitefulness exists (I've had people do some really petty swipes at me).
Also, we may not be as useful for some repos, as for others. For example, I was the original author of a project that I turned over to a new team, a few years ago. I could answer some basic "why?" questions, but would be useless for current stuff.
Would there be some way to "score" for that? Also, maybe some folks would only be useful for subsystems of an aggregate project, that might be in a monorepo.