Just a follow up on this...
While the shares accomplished what I was looking to do, I did notice that when a RP was relatively idle overall but other VMs in different RPs were making use of the extra resources, things would be a little sluggish on a VM in the idle RP at first, until resources were taken back from the overactive VMs in the other busy RPs. What I decided to do was create a small reservation on each RP so that some resources would always be reserved exclusively for use by a RPs own VMs. This allowed responsiveness on the machines to always be snappy unless there were other VMs in the particular RP that were consuming large amounts of resources. The ultimate goal here is to allow resource sharing but not allow one RP to impact the operations of another. This setup seems to be the best way to accomplish that.