I was in a circle where I lost all other roles (circle retired them by governance) except the role as secretary. I then decided to leave the circle because I then felt like an "external", having no other purpose in the circle.
My point is "external" Facilitators or Secretaries are possible - but don't make much sense.
a) it's a waste of resources (you are blocking someone from outside the circle)
b) it hinders the circle-members to learn those roles
c) people from outside won't understand the circle as good as circle members. Dealing with tensions, especially integrations, are difficult for an external Facilitator.
Therefore: +1 to clarify constitution to allow only circle-members