Hey Paul,
This is one of those moment when I wish I could a some spare cash in the back pocket, haha I am itchy to buy a ZN head as well as other few crucial components from different years and models and directly compare. I've been going back and forth checking compatibility on Partzilla and it can be confusing. It also makes me wonder if what they offer in the list is 100% complete, or just partial, being very detailed at it. For example, a starter reduction gear for an early model will show compatible with a specific ZN, but looking at that ZN it want show backward compatibility.
From what I see, the early piston doesn't have those holes, has a slight bevel, but doesn't have that distinct grove area machined above it. The other piston has extra holes, a bevel, and a machined grove. The holes open directly into the crank case and are partially in the grove, partially right on the bevel.
My 'bird brain' tells me that if whatever this does helping lubrication, it also removes oil more efficiently. Perhaps oil from the crank case splashes and gets into those holes giving improved lubrication, but also any extra oil has a place to collect (in that grove), and the bevel directs it to the holes and through the holes out - instead of being trapped (restricted passage) making it's way to the combustion chamber and disappear.
I think the key point here is the machined grove and holes drilled ON the bevel.
From what I gather reading and listening, I think the most common complain I've heard is oil consumption of these machines, not poor lubrication. Trouble with poor lubrication is secondary - due to low oil levels when people fail to check them.
I don't think the 20 hole design has anything to do with DFI or high profile. My A4 1982 carburetor model came with low profile 20 hole pistons. I think they already started implementing it earlier. For example, the 81 model shows two different pistons compatible. One 'STD', which I think would be standard, and the other 'L O/S' - I don't know what it stands for, but my feeling is this is the 20 hole piston.
I think if your machinist is willing to drill those holes that awesome, but I think he should also machine that grove. I have no idea how the engine would behave having holes and no grove.
Sorry for looong paragraph!
Stan