I have done this on a small scale before, like just a bit running down the side, but can you completely cover an iced cake without messing up the buttercream? I was thinking that the heated chocolate might mess it up.

You have to wait for it to cool to room temp. before pouring it, I think.

Ganche is chocolate mixed with whipping cream that was brought to just a boil and is poured over icing all the time.

Any time I do ganache I ice the cake in chocolate icing. Then pour the ganache on it. You can chill the cake to make the icing really cold, so that the ganache sticks in a thicker layer to the cake. But even if you don't chill it, it shouldn't hurt it.

Just did a 6 and 8 inch stacked Ganache tonight. The secret is to let the Ganche cool to room temp and it turns out great.