It sure looks like OS X uses PAM. In that case you should be abe to use the PAM-MySQL to perform any type of auth you want. Out of the box OS X uses a pretty straightforward PAM config for sshd:
$ cat /etc/pam.d/sshd
# sshd: auth account password session
auth required pam_nologin.so
auth optional pam_afpmount.so
auth sufficient pam_securityserver.so
auth sufficient pam_unix.so
auth required pam_deny.so
account required pam_securityserver.so
password required pam_deny.so
session required pam_launchd.so
session optional pam_afpmount.so
I haven't set up PAM-MySQL before, but assuming it's similar to other external database PAM modules, there will be a config file that you use to select the db credentials, which tables should be used, etc. Then you would insert auth sufficient pam_mysql.so
just before the pam_unix.so
line in /etc/pam.d/sshd
.
Theoretically that should be all you need.