Your requirements list is what is known as a Null Set.
It's looking that way. After more reads of this forum following the above posts I decided to bite the bullet, called our hardware suppliers and got them to send some thin rubber sheets and o-rings. I had a minion (showing a distinct pout) then cut the sheets and pack different ones into several of the spare keycap sets that I have, then stick some of them on some test Filco Browns.
The result is that while yes, there is reduction of noise it's fairly minimal, not even e.g. putting the Filco in the domain of the Topre, which is the noisiest keyboard in constant use at home. The added problem I think is that the weight of the O-ring or rubber sheet negatively affects the typing feel.
I have said minion slicing & dicing some much less dense solid packing foam at the moment for the final 'cuz I'm curious' test. He is *so* pissed :pound: I think this will solve or at least reduce the weight problem but it wouldn't offer any further reduction in noise, so I think we can safely rule this workaround out of the choices.