Directly on the cake is fine (if using on BC make sure it has crusted), it will not soak in 
It will dry very hard (its not like cutting through BC).
oh wait a minute, did you mean ice with royal icing? Ive never done that, I only use it for decorations.
I was just going to try to make a sunburst on a skateboard cake. So I would be doing it on BC.
So when they go to cut the cake will it crumble all over? (the RI)
My problem is that she wants the skateboard to be iced with chocolate BC. So I was trying to figure out what to do with the starburst.
TIA
Lu
It will crumble. If you are covering the cake with buttercream, you can do the detail work in buttercream too. Just use a small tip and let it come to room temp so it's soft enough to pipe.
I don't know if it's any help to you or not, but when I'm doing a carved cake (which I get the impression you are) I tell them, I'd be glad to frost it in buttercream, but if they want any sort of detail or realism to it, it needs to be covered in fondant so I can paint it. I put plenty of buttercream under the fondant so they don't have to eat it if they don't want to (they usually do).
Good luck!! 
Michelle
I've used Royal Icing with buttercream. How the RI reacts depends on the buttercream used I think.
The first time I did it I used IMBC which doesn't crust so the RI accents soaked up some grease and became somewhat soft. It wasn't a big deal since nobody touched them until time to eat the cake (and then they didn't care).
Since the RI soaked up the grease with the IMBC I now only use it with a crusting buttercream and I haven't had any issues (like Beth19 said though, I make sure it's crusted first).
I haven't tried putting it on before it's crusted.
I know the Wilton yearbook has several designs that use RI accents on buttercream frosted cakes.
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