I have tried stencilling on fondant, but can you do it on a crusting buttercream as well. How about a noncrusting buttercream like Swiss Meringue?
I works great on crusting buttercream but not on a non crusting buttercream since it will stick to the stencil.
I have stenciled 2 crusting buttercream cakes and it worked very well. Just make sure they have well crusted. I'm not sure about the Swiss meringue, but if you decide to try it, I would put it in the refridgerator for a while to make it good and solid.