I think your going to have a tough time with finding something to do that. What you need is an emulsifier. You might try addind a little bit of soap or detergent and see if that has any effect, but it might prevent the shellac from adhering to the wood. You could also try using a different alcohol that has more carbons in it; if you are using ethanol (drinking alcohol), try isopropanol (rubbing alcohol). I don't know about the availability of other alcohols though.