It's most likely the vinegar in the mayo that takes off the hard water deposits. So maybe you could just use vinegar diluted with water and it might do the same cleaning job. Then rinse the leaves with your purified water.
That being said, the mayo idea came from a fellow who grows and shows orchids, which are epiphytes and take in a lot of water and nutrient through the leaves. He's used mayo/water to clean the leaves of his orchids for years. You wipe it on with a paper towel, rub gently to get the crust off, then wipe most of it off again, and it takes the hard water deposits, dirt and dust away, and leaves a nice sheen on the leaves.
I was given a bunch of orchids that had been watered with well water for years, and they came to me all crusted and ratty looking. I cleaned off all those leaves laboriously with the mayo/water and so far, (18 months) have not lost a single orchid and only the odd leaf, in the normal course of things. I'm thinking the dirt and hard water deposits also block the leaves' ability to uptake stuff, so a little residual oil might be less harmful than all that crust and dirt. You could always wash off the residual oil with soapy water on a paper towel, too. But I didn't.
