What about a mod that contains a new furnace which smelts ores twice as fast and has 5 slots to smelt ores? So, if anybody has got free time... Well, it's just a suggestion as I have no idea how to do that myself. :/
Oh, it isn't even possible with modding? That's sad But I hope this gonna change in the future. Thanks for your answer!
Well - it is not possible through the standard JSON method. It is possible if you know how to use LUA - though it doesn't use interfaces properly yet