What I did was trim a thin aluminum disc (thinner than the compressed head gasket thickness) that fits securely into the extra oil return hole in the metal head gasket. Then I used some RTV silicone as a sealant, applied to both top and bottom of the disc, and I installed the it and the head and torqued the gasket before it cured to use the silicone as a sealant. Maybe that was a mistake, and I should have let it cure and install it compressed, because like I said there was a slight amount of coolant weepage when the enigne was cold. But it actually worked quite well when the engine was running, it always perfectly sealed and held in full cooling system pressure. I think the problem was that either there wasn't enough of the resilient material left for the thickness of the disc that I made, or it wasn't under enough compression to conform as heat contracted the head and made it move away from the block. Or maybe something else was going on, I don't know.
I didn't experience any problems from the plugged cooling passage. I figured it couldn't be that critical because it is not present on the hydraulic lifter motors.
Hope this helps. If you try something, let us know what you do and how it works!