Ryan --
GET /api/v1/accounts/:account_id/admins will list all of the users who have been assigned an account-level admin role within that account. For example, I have a "Department Chair" and "Department Member" role for each of our departmental sub-accounts, and listing admins will list all of those users.
It sounds like what you really want is a listing of folks based on their course enrollments within the account. Something like "all the teachers enrolled in courses in account X".
I've got a PHP script that lists teachers by term, but the general idea would be something like…
GET /api/v1/accounts/:account_id/courses
and then loop through those results, querying each course:
GET /api/v1/courses/:course_id/enrollments?type[]=TeacherEnrollment
or
GET /api/v1/courses/:course_id/enrollments?role[]=My%20Fancy%Custom%20Teacher%20Role
It _feels_ like there should be a better way, but I'm not sure that there is.