Chocolate mousse for frosting?
My favorite chocolate mousse recipe uses gelatin as a stabilizer.  I've used it as a filling between cake layers and it works really well.  I don't see why you couldn't use it on the outside of the cake as well.

You'll want to keep the cake refrigerated.  I don't know exactly how long it will remain stable, but I've kept it in the fridge for several days without a problem.