Good work Alan! I’m currently experimenting with materials for cushioning the keys. A layer of soft leather under 1mm felt works well, but I’m sure there’s more modern ways available now, such as your foamy gasket/silicon tape.
To take your idea a bit further, you could try fixing gasket material, leather, or other sound deadening material to the pads of the keys as well. This would have the dual effect of shortening the key depth and also absorbing some of the sound as it leaves the instrument