If you have a crusting buttercream and let it set up well, then very lightly crimp, it will work. I have done it before...sorry...no pics. It looks best if you smooth the buttercream using the Viva papertowel method which makes it look more like fondant.
Great question. Do you have a square, circle or whatever of styrofoam? You could try covering it with tin foil, frost it with BC, smooth it, let it crust and crimp away. Try several amounts of pressure until you find the one that will work and practice until you feel you can duplicate the pressure on a real cake. (I have not tried this but if you were told it can be done...)
And I really wanted to do some crimping...new toys anxious to use them....that kind thing ya know! 
Can't believe how quickly I put out of my mind that the customer wanted buttercream. Sorry! Ok, how about some thin layers of cake, rather than one regular cake. It would give you more surfaces to practice on. You really only need one layer, you can scrape off the icing and reapply a new layer to practice on. (Use the rest of the batter for cupcakes for the family, you can crimp those, too.) I hope this proves to be fun and profitable.
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