As of the latest snapshot 13w03a, the new hopper block is able to interact with furnaces properly. The mechanism it uses to identify the slots is not too different from the sided inventory mechanism. In fact, the Furnace and Brewingstand tile entities are now implementing an IInventory subinterface which actually IS ISidedInventory in all respects (if you don't believe it: the class is obfuscated to "lm").

 

The problem is: the forge version processes fuel from the bottom, output to the sides. The new vanilla version does it the other way round, as the hopper can only extract from the bottom. So I'd like to know what the forge devs will do when 1.5 comes out. Will you override the vanilla version (which would break the hopper) or will you adapt to it (which will break many existing builds).

Have you seen thermalexpansion? It can have its inputs and outputs changed.

I highly doubt that the forge devs will implement this for the vanilla containers.

I plan on tossing a config flag to revert these specific things.

And then completely dumping ISidedInventroy from forge.