the constraint to have access to text password or HA1 format (md5 over username, password, realm) comes from WWW Authentication mechanism which is used by SIP.
Writing something different in kamailio would be possible (it is open source), but you don't have phones able to do it and provide the adequate details.
Are you sure? You’ve a Radius backend for example, Radius don’t allow you to access to clear text password, it isn’t?
The only www authentication mechanism who must have access to clear text password is the DIGEST auth. But if we consider using SIP over TLS, we may be able to use BASIC authentication…
What do you think ? Does the authentication kind is negotiated during the communication?