Yeah sure, go for it.
If you want to be really realistic for the 14th century Europe, which is Toady's target setting IIRC, then it would mostly only be bloomeries, no blast furnaces. Which would require a good bit more work, but is doable. (note that pig iron is truly a waste product in that case, which it was):
1) You would want to add another reaction along the lines of "hammer bloom into iron" that takes one of the bloom_iron bars and makes it into an iron bar, for just one fuel (no extra carbon needed)
2) If possible, you would want to require steel armor and weapons to take a mixture of iron and steel. They would actually have been mostly iron core with steel casings forged onto them. Given the quality of steel at that point in history, this was stronger, because the softer core absorbed shocks without shattering, while the case held the edge and resisted weathering. But I'm not sure if this is worth it for modding. One semi-realistic compromise would be to have the forge make "un-hardened armor/weapon" and then allow an extra carburization at the bloomery to make it hardened armor/weapon. I don't actually know if that'd even be any easier, though.
3) Remove the smelter building entirely
4) Make your own custom smelter building copy for lower temp metal reactions only.
5) If you want to be really obnoxious, you could require the standard flux that is actually used for copper smelting ---
iron ore. Lol! Or to be somewhat fairer, allow high quality copper with just a little bit of iron ore flux (something like 2 copper ore + 1 iron ore = 8 pure copper), or lower quality copper without iron ore flux, which is a separate metal you can still make weapons and stuff out of, but modify the physical attributes to be brittler and weaker.
6) Tin and lead don't really require flux, nor does alloying already smelted tin and copper.
7) You should NOT be able to make bronze out of only the ores directly, like in DF.
Gold or anything else that comes in native form shouldn't need any flux or anything, since it's not actually ore - it's just melting nuggets into bars, at low temps. A campfire can "smelt" gold nuggets.
9) You could possibly consider arsenic-containing ores yielding stronger bronzes (terahedrite, stibnite), which is discussed in much more depth in another thread active now around here somewhere. You would in this case want 3 types of copper and 3 types of bronze: impure-non-arsenic copper, impure-arsenic copper (both weak themselves), pure copper (stronger), impure-non-arsenic bronze (only marginally better), pure bronze (better still), arsenic-bronze (strongest, not as good as steel)
One way of making it easier to mod all the smelting stuff might be to remove smelters and then add custom reactions to the kiln building, which the game allows you to do. And mildly realistic (kilns
would get the job done and may have been used earlier in history, though it's pretty anachronistic for medieval Europe).