Wow, I just read through the posts I didn't read and I completely said in my last post what has already been said.
I was going to argue that it requires the same amount of fuel to melt an ore into liquid for bar making that it does to melt metal items into liquid for bar making, but having them melted in batches does make sense the way you describe it. Although accurate, you're over thinking it as far as gameplay is concerned. All you need to do is expand the melting process (melt breastplate, melt warhammer, etc) and assign a percentage of a charcoal/coke bar required to melt each item. Multiple melting jobs for one bar of charcoal. Don't ask me where the
percentage is for melting items into fractions of bars or where the
code is for having multiple bar usage, I'm positive Toady removed the latter from current versions. I also believe that Toady is already planning on doing this, but that's just a rumor that I made up.
Another way to do this that I favor is to have them extract (butcher) metal items into scrap, same way items turn into fraction of bars at a smelter they turn into scraps (fractions) that build up for bars but without the usage of fuel. Scraps store as a variable in a workshop (easier), and when you have enough for a whole bar you can melt them which would then require fuel. Only problem is I don't know how you would do that last process with fuel. Contemplating if melting steel should be more difficult. There's already a quarry mod that has the same reactions for stone, one could easily work off of that.
A simpler method for this would be to melt X (iron caps) into bars, #IF [{X(.1)*10>=1} THEN makebar {Y(iron)}] where X is the material size rounded to the nearest 1/10, and Y is the lowest value material type used. I wonder if that code is even still in use, oh well I'm tired and you (hopefully) get the idea. Only problem with that is you'd need to set the material size functions to their proper form. Still, should require less work than the previous methods.
I'd do it myself, but it's easier to buy wood from caravans. They need a reason to enter my trade depot anyways, cause it's sure not going to be for their cloth.