For now I have the rear remote adapter for the tractor and a pair of hoses long enough to connect to the splitter and will configure the hose ends to plug together when not in use.
I looked into it more, the rear spools should be the same set up as the loader spools, with open center and a tank drain, with power beyond for the 3 PH, IF you want the 3 PH to work while the spools are in use, otherwise they can be normal open center. Alternately there is a kit that connects directly to the power beyond with up to 4 remotes, electrically controlled and mounts to the ROPS.
Electric Rear Remote Valve Kit for Compact Tractors (Two Spool, Three Spool, Four Spool)
summit-hydraulics.com
Because it connects to PB it doesn't require a drain, it's essentially the same as a log splitter or backhoe. For under a grand, 4 remotes can be added, that just plug right in.
I guess at one point Kubota had a full fledged rear remote setup that had all the parts similar to what is used on the newer tractors, with the spools mounted to the HST, and controlled by levers. At least that is what it looks like in the service manual, but can't locate a parts diagram on them.