i did this with chocolate flakes on a creamy pav on Xmas day - had it sitting on a piece of baking paper, leaned it up on a fairly steep, then sprinkled. cos it was cream i couldn't press them in, but i made bugger all mess anyway. i think with a cake you could prolly press them in a little bit... just don't do it too much or they'll get lost in the icing and all the effort will be wasted!