I just bought 2 female Kubota couplers from Messicks. Info as follows:
Coupler 1/4 HN, Part number 7J612-66323 (formerly 7J612-66320).
They are 1/4" NPTF thread. You can use 1/4" hose, or 3/8" if you prefer. I plan to use 1/4" as control is more important than speed.
Cost is about $14 each, plus shipping.
It takes a bit of doing, but you can find the parts diagrams on the Messicks site. Look for the Parts diagram for the FEL, and then the FEL hydraulics diagram.
Not sure if these are the same as the factory rear remote couplers, but I would think Kubota would kind of standardize. You may be able to find the parts breakdown for the rear remote kit on the Messicks site also. I didn't look.