If you are modding in this way, you can as well add nothing->steel reaction anyway
Well, this is not exactly modding. You can do those changes in any vanilla game for them to take effect. You do not need to start game anew with adamantine at top of inorganic_metal.txt, which is the only modding I do lately. Adding reactions does not mean they will become recognized and used automatically in an old save. I found out, if you mixed labels with some older recognized reaction, then this reaction will work even in a vanilla save. This is not modding, though done same way. Those are vanilla game's exploits, as they work in vanilla game.
TBH, all my latest embarks have been modded with adamantine pushed to the top of the inorganic_metal.txt's metals with waffers removed. I just can't deal with weird and confusing mechanics of extracting this material through clothing stockpile. You don't get too much adamantine from small map embarks anyhow without smelting exploit, which is an integral part of this game for years now. See? Dwarf Fortress is not a game, which encourages exploit-free play. Some projects are even based on existing still exploits.
Though you gave me an idea for 2 reactions. I have no idea if they are even possible.
1. In press: 1x500 coins (item gets consumed) -> 500x1coin (creates 500 unist of 1 coin for smelting maybe?)
2. In press: 100x1coin(100 items will get consumed. Will they even fit the press?) -> 1x100 coin (for storing maybe?)
3... similar reactions. This way the press will become the bank building I was suggesting in "suggestions" thread. How about 1x1000 coin (from 10x100coins) or 1x1000000coin?
What do you think? We could get banking into DF, before Toady makes it so.