Currently, neither HostBot nor the Co-op maintains information about who is being actively mentored (or not). We have more or less assumed that mentors will preemptively make it known when they are unavailable, but that may not always happen. This can cause learners to get matched to mentors who appear available, but actually are not; this circumstance has actually occurred during our pilot. Active mentors have expressed concerns in not having an straightforward way to detect when mentorship does not begin despite a match (see this thread and this thread)
One solution to this is to initially put new learners in an category of "not being mentored" initially. Once HostBot matches them to a mentor, we can direct the mentor to physically sign their name in a "mentor" parameter on the learner profile, which removes them from the category. We can also display this signature on the learner profile for visibility. I'm also open to other suggestions, so let me know if another approach might be better here.