what other options for frosting under the fondant besides buttercream and ganache??
thinking 3 or 4 tiers....
You would need to have a dam of buttercream or ganache to stop the fillings oozing out. And of course keep your cake refrigerated at all times if the fillings are perishable. This may or may not cause you problems with the fondant sweating.
As for what to use between the cake and the fondant, I haven't heard of anything other than marzipan (stuck down with boiled, sieved jam), buttercream or ganache.