[OpenSER-Users] Modify $au to lowercase

Hello, is there any chance to modify the authorization user name? I have the following scenario. We do allow only lowercase usernames when users are registering. So the md5 password is created and stored based on lowercase letters. When a user receives his account data he might enter his username in camelcase in his user agent. So when he tries to register, it fails, as the md5 hash based on the camelcased username is different. I tried to use the perl module, put the username to lowercase and put it back to OpenSER via avp_pushto. But Openser only allows $ru,$du,$br to be modified. Do I have a chance to modify $au? Best regards Peter