You can but first make sure you let the BC dry very well. I don't see why you would want to do this when you can just pour the ganache over the cake with out BC.
I have to agree that it would be sugar overkill.
Somewhere in one of the old post is a Chocolate Explosion Cake iced in Chocolate Buttercream, then covered in Poured Ganache. I have made it and was told that it is very good. I have also read that you are supposed to put a layer of BC under poured ganache, this is what makes it smooth. If you are using a whipped ganache then no you wouldn't.
It is probably a matter of personal preference either way, I am only writing what I have read from other post.
Wilma
I recently made a groom's cake with buttercream and ganache (poured) and it was not to sweet it was so yummy and the cake was chocolate. I was glad I was invited to the wedding to taste it 
And I agree with Katleg, frost the cake with bc first to give it a smooth look.
well thank yall! i think ill try it both ways with BC and without.. i just thought it would give it a smooth look...i didnt think about it being too sweet, but i love sweet stuff so i dont know! i love posting on here everyone is so helpful!javascript:emoticon('')
Very Happy
I have always used chocolate BC under my ganche, works fine! 
Everyone has different tastes. Ganache taste so wonderful on it's own that for me adding a pwd sugar buttercream would detract from the taste of the ganache and be too sweet. The difference would be like eating a chocolate cupcake or a eating a dingdong.
I have used a think layer of real IMBC just because I wanted a hit of vanilla. IMBC is not sweet. As long as your ganache isn't runny, it should look smooth once it sets.
Good luck with your experiments!
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