What would be the best way to adhere a large flat piece of modeling chocolate to the sides and top of a buttercream-iced cake (already crusted)?

I use candy clay all the time, but have never tried to attach really large pieces. The weight may be a problem.

You could try using buttercream or maybe even try painting a thin coat of corn syrup to the back of the piece. Don't press too hard when applying the piece or you will cause the icing to bulge out.

HTH.

Kim