I wonder if anyone here has actually performed such a “hammer test.”
Me obviously

After a friend (and later a prominent teacher) demonstrates that to me. Just get a hammer and try what I suggest for yourself on a decent grand piano - it's not hard at all.
I remain unconvinced, and only an unbiased scientific experiment conducted with appropriate measuring instruments would persuade me that any additional volume is achievable through unconventional means.
What, with a decibel meter or some wave form? You do realise that when it comes to perception, we're not just talking about volume, but the kind of sound, which cannot be picked up by those measure instruments? But I guess if you believe more in measuring instruments than what human perception can pick up, there's nothing I can say...
I’ll continue to believe that maximum volume is already via the usual apparatus alone (i.e., torso-arms-hands-fingers).
Yes, I absolutely purport that this is the apparatus you need to get the most sound out of the piano. So I guess you will agree that the weight (and perhaps also the kind of weight) you put on the keys will determine the sound you get out of the piano? Hence partly the purpose of my suggesting such experiment with a hammer, constrasting the sound with using just the finger, and finger alone.
I'm not sure! I don't even know how harshness figures into the physical properties of sound or how it is quantified. Louder does seem to equate to harsher (on pianos, in any event), but I wonder if the harshness component isn't also maxed out in a conventional fortissimo.
Yeah, I do think it gets rather subjective here. Generally when most people I know say the sound is harsh, it's when the volume is loud and the articulation too aggressive (as a result of putting so much weight without releasing it when depressing the keys very quickly). The sound usually doesn't ring or sustain very well, but instead you get a very substantial initial hit when the hammers strike the strings.