I think I may have had an interesting idea. Since o-rings dampen the sound when bottoming out pretty well, there's still the problem of the key making this high pitched sound when snapping back up.
So I opened a Cherry MX Black and looked exactly which parts cause that sound, it's the stem's sliders bouncing against the upper part of the housing, just two small spots. To dampen this sound, I took the housing (since that is the non moving part) and put some glue on the two spots, then I took some fibers of a thick sock, making two small fluffs and glueing them inside.
I probably should have taken some different glue and made this a bit more properly, but this more a proof of concept.
Here are the results with a non-mounted, non-lubed Cherry MX Black switch (I am a bit sick but it's not that bad):
Vanilla switch:
http://vocaroo.com/i/s17Q2clQt4dZVanilla with blue o-ring:
http://vocaroo.com/i/s0dZIp4zPwMEModded switch:
http://vocaroo.com/i/s0dh19b7yrDpAs I say in the last recording, it feels weird. Maybe that's the glue transferring a bit to the sliders, but that's rather my fault with the "wrong glue". However, it is very very silent.
Should I do more experiments with this?