Yes it mentioned firming piano hammers in the article. From what I remember, a piano hammer is a shaped piece of wood (or several?) with a leather strip around the striker part? What is the difference for you between hardening and softening the hammer, and how would it be done with this .. is it penetrating? (acetone base would enable that, it is used for carrying chemicals through a surface). Could you soften the hammers by replacing the leather strips, or soaking them to loosen & expand the presumably compacted fibres?

In my wider life in the UK, speaking to people associated with pianos (from a piano tuner, to school premises teams), it is often not worth the commercial expense to repair old pianos unless they are of particularly good quality or have some sentimental value.