Matt -
It shouldn't be that complicated, or am I missing something in your question ?
SnapVault has always been about being able to retain Snapshot Copies on the destination for much longer than you retain them on the source.
I'd do this the same in Cluster Mode as in 7-Mode.
For an example:
Create an update policy on the secondary with '-name monthly' using the 'job schedule create' command.
On the source create a monthly snapshot policy with 'volume snapshot policy create' and use the 'snapmirror-label' parameter to specify a named label (monthly) for snapvault to use.
Use 'snapmirror policy create' on secondary to make a monthly vaulting policy.
Add a rule on the secondary to retain the monthly snapshots with 'snapmirror policy add-rule' and 'snapmirror-label monthly -keep 120'.
See also the Data Protection Guide documentation:
https://library.netapp.com/ecmdocs/ECMP1366833/html/frameset.html
This would retain the monthly snapshots on the secondary for 10 years.
I hope this response has been helpful to you.
At your service,
Eugene E. Kashpureff, Sr.
Independent NetApp Consultant, K&H Research http://www.linkedin.com/in/eugenekashpureff
Senior NetApp Instructor, IT Learning Solutions http://sg.itls.asia/netapp
(P.S. I appreciate points for helpful or correct answers.)