This would require every Forge mod maker that uses Bronze to come to a consensus on the rarity of copper, rarity of tin, spawn area for each, and their conversation rate to Bronze. Having seen how fiercely individualistic and unwilling to allow anyone else to "compromise their vision" some of the mod makers get, I don't think that is happening any time soon.
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It doesn't really.
All forge needs to do is enforce "strict unification" for ores: It would register only one type of copper: first-come-first-served or via config. At any rate, the first mod to register copper (ore, ingot, dust etc.) (for example) gets it, every subsequent mod that tries to register copper will get back the first mods copper and believe it to be their own.
Forestry, TE, Tic and IC2 can then all implement their own configurable rules for spawning copper - it doesn't really matter as all the copper ore generated would be the same.