Dear Evan,
AFAIK when TLS is configured, syslog-ng behaves
differently, depending on whether we are talking about a
source or a destination.
A destination will perform subject CN checking to verify
whether the server is who it claims to be.
In case of a source however no CN checking is performed,
only the validity of the certificate and the certificate
chain is checked, depending on the peer-verify() option.
Despite this, it is possible to define a list for the
option trusted-dn() and/or trusted-keys() so that the source
will only accept connections from clients with the specified
certificate parameters (Distinguished Name - trusted-dn(),
SHA-1 fingerprint - trusted-keys()).
Best Regards,
János