The VMware host storage is a little limited
If you don't have enough space to keep all the VMs that you want to replicate on the DR cluster then the replication will fail as there will be nowhere to place replicas' disks.
Why stay away from RAID 0?
Because a failure of a single drive will destroy the whole array with all data on it.
Is there a way to advertise some of my 16TB free space to the cluster so that it can be used for replicated VM storage?
You can create a separate LUN and present it to the cluster via iSCSI and create a datastore dedicated for replicas only, however I would not recommend doing that either because, again, it makes no sense to keep anything important that you rely on (I believe that replicas truly are) on RAID 0.