Re: Plugging a tone hole.
Well it sounds like the pad needs to be replaced anyway, so try putting a couple pieces of duct tape on the pad. Just trim around the key cup, actually adding height to the pad. This might help stop the leak and actually make the key fuctional.
It's a cheap fix, and although I have never personally tried it, I think it might work. I'd definately give it a try. Worst that could happen is that you might have to replace the pad, but if it's dried out anyway....no big loss!