Don't do it, you'll look crazy. Everyone will be able to tell you used Kool-Aid. It's kind of cute/funny on little girls playing dress up, not so much on a grown woman. I like the idea of color lacefronts or clip ins.
Well, don't some people add pectin to their henna to make the rinse out easier? Couldn't you add red jello and have the same coloring effect as adding Kool-Aid?
One of my old homegirls used the bottle of koolaid with the clown on the front (back in the day) and it gave her the most beautiful bordeaux color. It was kind of like henna in being that it didn't change her hair color (jet black) but you could see it when the light hit.