I have seen this cake popping up all over Pinterest – it is very stunning! Royal icing should definitely work, but I wonder if piping gel would make things easier. Put a thin layer of piping gel on a section and then load it up with a handful of dragees. I would test it on a small cake before I started on the real one! Good luck, please post pics when you’re done!!
Jenniffer White, Cup a Dee Cakes - http://cupadeecakes.blogspot.com