I just bought a Harbor Freight 3PH log splitter with an open-center valve. Yes, I have to bungee my remote lever fully open, but it's so easy to do that it's not even a distraction. The only thing I have to remember is that the the 3PH will not lift if the remote valve is open because it is before the 3PH in the open-center system.
I'm not sure this will work, but why don't you see if you can bungee the lever open on your splitter and then use the tractor remote lever to operate the splitter? If that works, you'll not have your hydraulics lifting the relief valve like you would if you try to use the closed-center valve on the splitter. You might even rig an extension lever on your tractor's remote lever so you could operate it while standing at the splitter. I think you may eventually want to replace the splitter's valve, but at least this might make your splitter work for now without damaging your tractor.