Problem:
Authentication for requests performed by the remote API do not go through Seraph, so they can't take advantage of Seraph authenticators. XML-RPC and SOAP authentication requests are checked directly against the user-management framework, and tokens are assigned directly by the remote API subsystem. [src: http://confluence.atlassian.com/display/DOC/Understanding+User+Management+in+Confluence ]

The answer of the support is:

If you really need Seraph to be the authenticator, you can either:

• Modify Confluence source code - You'll need to look into the ConfluenceSoapServiceDelegator
• Write a RPC plug-in which extends (and overrides) the functionalities of the class mentioned above
  Perhaps you would like share your concern and raise a feature request at our issue tracker.

Any help for doing this?