The slots in the first two pictures are there to help prevent the plate distorting, the last one is as used in BSA six spring clutch's and are there to push oil out from the plates as they should be used dry.
As you say plates are cheap enoght to give it a try, the first two pictures I think are Velocette, and to be honest I have used both them and plain plates without any difference in the way the clutch works.
Cork has no oil drag; they run dry, as with Ariels, although some do work in oil, the manufacturers say to keep oil off the ones which are supposed to be oil-free.
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Brake rotors have holes to get rid of water, the clutch plates are better with more surface area rather than less. I've come across people in the past suggesting roughing up plates with corse sandpaper but this is also a mistake as the smoother and shinier the plate the more surface area and the more friction.
Main reason for holes, slots or waves in brake rotors is to vent heat, increase bite, vent gas, and as a side benefit aid in ejecting water, and even mud/dirt. Look at dirt bikes as an extreme example. Any combination of waves, slots and or holes. More leading edge, increased "bite", better at ejecting contaminants etc. Simple facts.