I was wondering if I can use Antonia's Royal Icing on a cake to fill in a design. I was also wondering if I can use it on wax paper and if it will dry stiff enough to move the piece on a cake.

Has anyone tried it?

BUMP!

Thanks for bumping!
I PM'd Antonio and she says no because of the oils in buttercream. I completely forgot about that!

You can't pipe a big design directly onto a buttercream cake and have it dry hard, no....but you can pipe the design onto parchment paper (thick and large enough so that it doesn't break!), let it dry for a few days if it's big and then transfer it onto your cake, yes. Royal icing isn't great in the fridge for extended periods, it tends to soften up especially when condensation occurs as the cake is coming back up to room temp from the fridge...so maybe put it on the cake at the last minute?

Do use parchment paper for the piped design though, not wax paper! I made the mistake of piping royal icing snowflakes onto waxed paper for a cake and they were still not dry after 2 days. Waxed paper didn't work nearly as well...and it wrinkles easily.