What we did in the past was create a mailinglist with all the potential coaches, then send a doodle and see who's up ... but that was a lengthy and error-prone process, that didn't allow people to join easier later.
What I'd recommend doing now, is like we did for flossie here: Once you enough coaches that you can definitely run it, post the decided details in the events-category asking for coaches to just quickly mention there, they'd come. This also has the benefit that they (and only they) received updates when you post something there. Then you'd just link to that conversation in emails or twitter asking people to join. Or – as in this/your case – put it into the description/send it to the people, who already joined, asking them to post there if they are a coach.
Normally, we do not require our coaches to sign up on the meetup event, so that number reflects the number of learners. Recently we started adding "coach for X"-custom-titles to known coaches in the community and asked them to join the meetup events that have a more lose structure (like the learners meetup or continuous learning groups) so others can see, that there'll be coaches too. For workshops, asking learners to sign up via meetup and coaches on a discourse thread, seems to be the best way imho.
Hope this helped