This not a but this is an enhancement as original requested.
General, it is not possible to just add the -p flag because this could brack the replication on non-ZOL systems.
So this must be an optional flag.
I understand, I wish someone from PM would update the "bug" request with this data.
I would also suggest, that you list this limitation on the manual: https://pve.proxmox.com/wiki/PVE-zsync
because we normally assume that synchronization does actually sync data along with it's attributes.
Or even better, make -p flag automatic, when ZFS is used. BTW pve-zsync uses only ZFS, so it could be by default there, no if statements required.