Yes, using a sieve filter is a great idea and just works. This way everything can be managed from cluster-admin UI and just NS8 base apps are used.
I created a user fetchinguser which is granted access (full control) to the public mailbox “publicmailbox1”:
In Roundcube I created a sieve filter for the fetchinguser to move all mails that are no duplicates to the publicmailbox1:
In Imapsync I setup a task that syncs mails from my mailserver to the fetchinguser mailbox and enabled sieve processing, see also Imapsync — NS8 documentation
Now the mails from the mailservers “markus” mailbox are synced to the public mail folder publicmailbox1.