Mike The closed center plug should just have a longer section with an oring the seals the return passage so the oil does not free flow when the valve is in neutral. You can just cut it off so the internal port is not sealed. As for the return you can easily just hook the return to where you wanting too. The only issue would be IF you used the remotes or three point at the SAME time as the loader valve there would be pressure on the return side of the loader valve. This could make it not function as both sides of the valve/cylinder would have pressure on them. This would ONLY be true while the oil flow was going to the remotes or three point.
I have seen several loader on Oliver tractors hook up this way and they were used that way for years.