I investigated this option a little bit:
From the side of SASL, you can use the PLAIN mechanism to authenticate against system accounts. The best option is probably PAM and
saslauthd. But "shared secret" mechanisms are not possible (unless your system account password are stored in clear).
From the side of Subversion, the SASL notes of 1.6.4 list under the "Known Issues" that they outright disable the PLAIN mechanism in both ra_svn and svnserve because it sends password in clear text. In the notes of 1.6.5 they lifted this restriction and explain that you should take care to protect your network communication. They probably made this change because they are aware that it is needed to use
In particular, this problem affects users using the saslauthd daemon to
authenticate users, because that method (=PLAIN) only works with plain text passwords.
So in conclusion, I think you can get it working, but only for a very limited number of configuration: client and server must have Subversion 1.6.5 or higher with SASL compiled in.