I tried to implement a non MikroTik AP, but I had to give up due to this error message.
"unknown authentication algorithm for user <username>"
I guess a non MikroTik AP will not work, until the RADIUS client will be able to send/receive all attributes supported by MikroTik User-Manager.
Edit: I guess that the following thread pretty much answered the question.
http://forum.mikrotik.com/t/using-usermanager-as-radius-for-other-ap/20825/1