I was thinking of doing a crumb coat for a chocolate cake I'm making, thinking it would help hide the dark cake better. Does this work? Should I make a regular ganache or whip it? Any problems with buttercream sticking to the ganache?

I do this a lot, under both fondant and buttercream. It works very well. It tastes divine!

I will often use ganache as a crumb coat under fondant and buttercream. You have a light hand and plenty of buttercream for the final coat as the ganache will meld with the buttercream and could show some smear through. But using enough buttercream will prevent that.