I have deployed DMASOFTLAB radius manager at various networks which have this option. If a user forget his password, he can retrieve it by clicking on FORGOT PASSWORD link, After submitting the mobile number, user will receive a code on his mobile number (If he have defined valid number while registering the account or by admin. Then after entering the code , he will receive new password.
You can see its example here under heading of "Howto Recover User Password via FORGOT PASSWORD link at user.php (ver 4.x or above)"
http://aacable.wordpress.com/2012/11/26 ... iguration/
I am pretty sure it can be done via customized backend scripting with some php frontend. User must have valid mobile number entry in radius table. so the script can verify his mobile number and can change the password this particular user and then it can sms/email him the new password. for sms you can attache any mobile to your radius box, for email you can configure gmail as relay and send it via script.