pam svnserve over ssh tunnel

[chiasa.men]

Goal:
User A opens a port forwarding tunnel via ssh to a svnserve daemon.

Since the tunnel exists User A can do e.g. "svn co svn://localhost/repo".
svnserve shall recognize User A as such without the need of reauthentication 
or providing other credentials.

Is there any way to write a pam module that recognizes the tunnel when the svn 
command hits the server?

First I thought the pam_ssh_agent_auth module could do what I want but it 
seems that the tunnel is not even forwarding the SSH_AUTH_SOCK.