Hi Andrew,
At the moment SMSQL's integration with PM only supports SnapVault relationships or "Remote Backups". So there is no way you can use PM to get SnapMirroring done from SMSQL. This can be done directly through SMSQL backup plans themsleves (as you may already be aware of). The only point is that you'll have to create the SnapMirror relationships manually.
Regards,
Sourav.
Technical Marketing Engineer (SMSQL , SMMOSS , VFM)
NetApp.