Does the API_UserRoles call not work for this? Need more specific information beyond user id, name, and role and role id? Just not formatted correctly?
Another option may be to use the API_GetUsersInGroup. This returns name, email, and if they are an admin or not. If you were to add all new or active users to a group this may work for you as well.
I know those are not full solutions but they might get you somewhere. You can keep an eye on the new RESTful API and see if they end up adding something better.
https://developer.quickbase.com/