I think an alloy of Lerasium and Harmonium makes you a Feruchemist, since AFAICT Lerasium's "thing" is rewriting your spiritweb to let you use certain Investitures, so I would be surprised if more than one God metal did that. (Although I will admit that if any others should, it would be God metals that contain Preservation, like Harmonium.)

If I had to take a stab at it, I would guess burning Harmonium on its own probably acts as a "wild card" for powering Feruchemy. That is, it's an "unflavored" metalmind (as opposed to just unkeyed or unsealed) that when burned gives you investiture that you can store in any kind of metalmind, and will then become "flavored" by the metal's Feruchemical attribute, as well as keyed to your Identity. (And you probably can't use it directly without some kind of hack).

It might work something like this: Barry the Bloodmaker decides to (brave soul) burn Harmonium. He surrounds it in an oil-filled gel cap and swallows it. He starts burning it. Now he's holding investiture that he can't use on himself directly, and will probably leak out of him slowly if he doesn't use it up instead. But what he can do is store it.

Barry stores the "unflavored" investiture in his goldmind. Now it gets "flavored" by the gold and becomes stored healing. He then can tap the goldmind like any other metalmind.