I mean, technically almost any nutrient can be used as a foliar.. The key is how you prepare it, and if that nutrient is water soluble .. You can't just mix up some mc, but it in a spray bottle and get quality results. You will need to add a wetting agent of some sort. I'm not familiar enough with the ingredients of MC to say it can't work.. But I can also think of a few issues that may arise as well.. Usually foliar sprays are of one particular nutrient, or used to correct/prevent a specific issue.. A calcium foliar for example. I don't see many full range NPK foliar sprays around.