You can make royal icing and add Glycerine to it.
Here is MissBaritone's recipe for it:
Add 1 teaspoon of glycerine to every 1lb of icing sugar. It will set hard on the outside but the underside stays soft. However glycerine must never be used on the coating of a traditional wedding cake if you want to use pillars to support the next tier as it has to set solid to support the weight
Hope this helps!!
I am not sure.
I tried adding meringue to my buttercream once and it altered the taste too much for me. I didn't like it at all.
You could just do it in buttercream and let them dry well.
I believe kitchengifts.com has a hard crusting BC recipe. I have never tried it out, but plan to very soon.
http://www.kitchengifts.com/airdriedbuttercream.html
Up your powder sugar. 
More powder sugar, more crust. More of your crisco/butter, less crust.
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