They show up on evil bay occasionally.
If I recall, there are 3 ways to 'modify' the behavior of the rockshaft cylinder while you're trying to use the other circuits.
One - a simple mechanical lock-up kit that locks the mower deck in the up position so that you can use the rockshaft cylinder strictly with the rear hitch. Far as I know that isn't going to help with the front circuits unless with that you can somehow restrict the movement (via the rockshaft adjustment) of the rear cylinder and force the hydraulic flow up front.
Two - there was a shutoff valve kit - that took the rear cylinder out of the loop. Not as quick to switch back and forth as the diverter valve but I think this will give you what you need. Should be very easy to re-invent from standard inexpensive hydraulic parts if it does what you need.
Three - the side-mounted diverter valve. That allows easy switching front-to-back of the shared circuit.
I know at least one person has incorporated a home-created variation of #2 - using a 12v electric-solenoid operated shutoff valve. That gave him a simple dash switch control to do what you need, but does draw a bit of juice to keep closed. Can't be any worse than the electric PTO clutch, so if your not using both at the same time, I wouldn't think it would be a problem.
In addition to the web site John mentioned, there's a lot of specialized expertise and knowledge on the older JD lawn and garden tractors on the weekendfreedommachines site.
