Creating new GeoServer user through rest api?

I've tried posting this xml:

    Jim

    password
    true

to these addresses:

http://localhost:8080/geoserver/rest/usergroup/user
404 - "Not Found"
http://localhost:8080/geoserver/rest/usergroup/users/user
404 - "Not Found"

http://localhost:8080/geoserver/rest/security/usergroup/user
404 - "Not Found"
http://localhost:8080/geoserver/rest/security/usergroup/users/
500 - "Server Error"
http://localhost:8080/geoserver/rest/security/usergroup/users/user
404 - "Not Found"
http://localhost:8080/geoserver/rest/security/usergroup/user/Jim
500 - "Server Error"
http://localhost:8080/geoserver/rest/security/usergroup/users/Jim
404 - "Not Found"

http://localhost:8080/geoserver/rest/security/usergroup/users/user/Jim
404 - "Not Found"
http://localhost:8080/geoserver/rest/security/usergroup/service/MyGroup/user
404 - "Not Found"
http://localhost:8080/geoserver/rest/security/usergroup/service/MyGroup/users
500 - "Server Error"
http://localhost:8080/geoserver/rest/security/usergroup/service/MyGroup/users/user
404 - "Not Found"
http://localhost:8080/geoserver/rest/security/usergroup/service/MyGroup/user/Jim
500 - "Server Error"

http://localhost:8080/geoserver/rest/security/usergroup/service/MyGroup/users/Jim
404 - "Not Found"
http://localhost:8080/geoserver/rest/security/usergroup/service/MyGroup/users/user/Jim
404 - "Not Found"

I've tried with and without the GeoFence plugin.

Is this actually possible?

I have succeeded in creating roles, usergroups, workspaces, coverageStores and raster layers from GeoTiff files, and dataStores and vector layers from shapefiles, but no progress on creating users.

I gather that some modules require you to change the admin password from the default, so I've done that too. My admin account has ADMIN, GROUP_ADMIN, and ROLE_ADMIN roles, and I've even added a USER_ADMIN role in the case such a thing exists.

I'm running geoserver-2.12-SNAPSHOT from 18 September.