Yes, mine is plumbed with a 75$ cheap open center valve I have bolted to the fender.
There is a plug on the side of the hyd casting, inverted pipe plug I think, anyway, that plug is in the path of high pressure oil after the relief valve, Normally that plug is left in, and oil travels up the side of the casting thru a port till it enters the top cover, then into a port on the small option cover, then goes back down into the top cover to the hyds mechanism.
What you do is pull that small cover, find the oil 'in' port on it on the underside of the small cover, tap that port for a pipe plug, like one of those inverted hex ones. install the plug from the underside. next, you find a place in the galley on the top side of the cover that is past that plug, and drill and tap for a hyd fitting.
I do have a couple pictures.
In the picture of the option cover with the port installed. that's the galley that also gets the plug, on the underside above the oil port on the tractor. IE.. the plug is on one side of the galley, and the port on the other.
The picture of the lower side of the tractor shows the port with the plug you remove to get hyd pressure. if you look close you can see the hyd manifold pipes on the underside of the tractor ( from the pump ).
A machinist did mine.
What that gets you, is that since the oil can't flow past the plug into the hyds, it must instead travel thru the hose you add coming out the side of the hyd casting, go thru your open center valve, then return to the top of the small cover, basically just bypassing that small plug. your open center valve is essentially invisible to the system when it's work ports are not active.. thus the 3pt works normally. You do want to choose a valve that can handle downstream presure ( like a PB capable valve ), because the 3pt lift is downstream from your hyd valve. It's not hard to find these valves, nor expensive.
If you ever remove the valve, you must do one of 2 things, either loop your hose to the fitting in the cover, or remove that cover, remove that plug, and then add a plug to where the port was installed. eaither way returns it to normal path of operation.