I am not a big fan of Foliar feeding, most of the time it is done at the wrong time of day and so ends up burning the leaves. It has been my personal experience that well prepared, properly nourished soil does quite well for my plants.
If you want to foliar feed; Steep 2 cups of grounds in a 5 gallon bucket, for 6 - 12 hours. Use to water and foliar feed your plants. The benefits of foliar feeding are the subject of much debate, but as always, let your plants be your guide. If they do not appear to be doing well after feeding, stop feeding.
Making a liquid fertilizer from steeping the used grounds is not quite the same as using left over coffee to water your plants. Left over coffee is more acidic and has other compounds in it that are removed from the grounds. Some acid loving plants enjoy the occasional watering with coffee, but they take their coffee black.
We love visitors, that's why we live in a secluded cabin deep in the woods. "Buzzard's Roost (Asnikiye Heca) Farm." Promoting permaculture to save our planet. you can call me Dr. Redhawk
Try one plant and see what happens, that's what I do. I have never tried coffee but I have done tea and noticed a stronger plant, sun tea, the coreopsis turned into a bush. I need to start doing experiments like that again, since we've had kids it's harder to do stuff like that and remember what your doing.
Unlike Bryant, I am a fan of foliar feeding. But I have never done it with coffee. Sounds Acidic. I don't think I will ever try it with straight coffee. As far as I'm concerned brewed coffee is for me and not my plants.