Is it possible to do a sponging effect over buttercream? Is so, how do you do it?

I would just have to do some trial an error on that one.....Try it without crusting, incorporating some spongy color, then waiting for crust and smoothing. I would try while waiting for the crust to get hard and then use a soft sponge with thinned buttercream, then smooth again after crusting, if a smooth finish is to your liking. Maybe someone here has done it, but I would just start practicing. Make a batch and just start on some wax paper using different techniques No need to wait for the cake!