I think the grease would probably soak through the ribbon. I read where people have used a strip of waxed paper around the cake. I recently used contact paper and stuck it to the ribbon.
When I have done ribbon cakes in the past, I just make sure that all the ribbon is touching the buttercream so that is soaks up all the grease uniformly and you can't even tell- well, its a bit darker than the original ribbon, but nobody knows that 
Here is a link that might help....
http://www.cakecentral.com/cake-decorating-ftopict-30203-ribbon.html+buttercream
Sorry, I don't have any personal experiance with this.
I used ribbon once on a cake and didn't seem to have a staining problem(but then it was a white and lace ribbon) I read somewhere that you can rub the ribbon with wax paper before you put it on the cake to keep it from absorbing the grease.
Good luck.
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