What is the organic way to prevent rhubarb leaves from having holes eaten from them?
Step one - if the holes are not particularly large/numerous, ignore them, you don't eat the leaves anyway unless you have a death-wish.
Step two - identify the pest - often a quick trip at night with a flashlight is the most effective method - day or might, look under the leaves if you don't see things on top. You can't control or manage an unknown pest very effectively.
Once you identify the pest, control options can be considered - hand-picking, organic-blessed poisons (Bt, rotenone, diatomaceous earth, ...) encouraging or importing predators (attracting birds, buying ladybugs, predatory wasps, etc...) Many control options will have side-effects (organic-blessed poisons can poison beneficial insects/animals too) and the goal is acceptable damage levels (see step one) not eliminating anything that might eat a hole in the plant.
It's probably wasps. For some reason they love to eat holes in rhubarb leaves and stems. Normally they use dead wood or hogweed etc to build their nests so why live rhubarb I'm not sure. Would be interested to hear if anyone knows.