I wouldnt count on the subframe having the hydraulics.
You mentioned adding 2 spools... I'm pretty sure Kubota taps into the power beyond loop. The hoe gets plugged into that loop & when its unmounted a jumper hose is used to keep things flowing.
You could plug into the ports on a spool. But the OEM spools for the Ls aren't detented (the OEM valves for the grands just stuck & leak internally by the way). You could just use a bungee cord to hold the un-detented valve open to make the hoe work. A little bit redneck looking , but functionally a perfectly serviceable option. That's the route I would go to avoid messing with power beyond plumbing.
http://www.tractorbynet.com/forums/hydraulics/295816-kubota-l3200-tnt-3rd-function.html documents my hydraulic project on my former
L3200. Those valves from FitRite weren't cheap, but were nice. The dealer has replaced the rear valves on my new
L4060 several times & the best of those valves isnt nearly as good as those Butchers.