I can't find the vid online, but Will Ferrell played the cowbell with Queens of the Stone Age when he hosted a couple seasons ago. So maybe you were thinking of both performances and got them mixed up? Either way, the QOTSA performance was hilarious. I'd look for that too.
So, when I first saw this, I thought is was... just okay (it starts off a little slow). But since then, I've watched it at least 20 times, and I'm now convinced that it's pretty brilliant. They really did their homework.