I know it's not a good idea to use the humidity tray water on the plants. But, sometimes, I do give them a little bit, if I don't have my watering can handy. I guess this'll be a good incentive to stop doing that. Win/win.
Does 1/2 teaspoon/quart of water sound too much/too little?
I'm assuming you mean something like Ivory, a non-detergent?