Has anyone ever tried to make a shell over icing for a base for fondant so it can be lifted off the cake easily?

That's an interesting concept, but I would think you would have to use a fondant gumpaste mix or adding Tylose to the fondant.. to harden the fondant, and then maybe creating it over the pan you used to bake the cake and letting it dry.  Curious as to why  you would you want to do it anyway.

Are you making a fancy fondant plaque that will be kept after the cake is et?

Just lay some plastic wrap under the fondant.  At the point of serving, the point of the cutting knife can pop the fondant plaque off in one piece.

If you use a thin layer of royal icing, you will have to let it dry before anything goes over it. 

I was thinking the royal icing would be able to smooth nicely for fondant to have a smooth foundation for a sculpted cake. Also, the client wants frosting, not fondant, but the "look" needs to be accomplished with fondant. She asked if I can make it so that the fondant can be removed and the frosting is still thick.

That is an interesting idea... Plastic wrap! I will try that... and also a chocolate ganache... and also a gumpaste shell. I would like to preserve the fondant piece for the recipient. That would be a nice touch.

If the client wants the "look" of fondant then you could consider modelling chocolate.

Have a look at www.wickedgoodies.net because her work is all modelling chocoalte.

Lol, if someone asked me that I would be like 

Tell her if she doesn't like fondant, she can eat around it.  And just because she doesn't like fondant, that doesn't mean that everyone at the party hates it.

OMG I would say get a new client.....this would just be nerve wracking for me and I would spend every minute of the day thinking about how'd I do it.

I don't think it would be worth it for me.  

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Well, the thing is... I love the challenge, so it gives me something to be creative about in my spare time. My clients are patient enough to allow me to build my craft at their expense, so I will try anything they come up with!