Ithaca I'm saying that (1) I think finger velocity alone (measured at the relevant location/point in time) is insufficient to completely determine the final hammer velocity
Yes, I can agree with that. I'm just saying in the end the result is control over the individual note volume and that is linked to timbre not independent of it.
Ithaca (2) varying finger "gestures" on a key that seem similar in volume/force can affect the timbre of the note, because the gesture can affect how long the note is allowed to sound before it's damped. In my own piano, with the wonky D note, it's very clear: the extra metallic/nasal/brassy/blaring resonance has a much faster decay than the primary resonance of the note at certain volumes, so at those volumes, when you sound the D, the timbre of the note changes over time, if you allow it to ring for long enough. At the worst (seemingly) constant volume, letting the note sound for about a full second makes most of that annoying higher-order resonance go away, whereas a maybe 250 ms press will give you maximum obnoxiousness. For this one note, it's like you've combined a piano with an out-of-breath kazoo player, but for the little bit when the kazoo is going full force, the timbre is decidedly unpleasant.
I see what you're getting at but I don't think this gives you as much control as you might think. The time until releasing the note is basically what pianists call articulation and depending on how fast the music is going you don't have that much choice in when to release a note. Maybe you have a bit more leeway in slow music.