Just had a thought...
Once you've got the yellowed plastic cleaned up, might it not be possible to apply a coat of laquer to protect the plastic?
You can buy matt / satin finish laquer from most model shops, but I'd suggest going for a high quality brand that doesn't yellow upon application. Airfix laquer is pretty shocking for this, to try something like Tamiya etc.
You could also laquer the individual keys, thus protecting the printed characters and hopefully preventing the kys from being worn shiny over time.
I've never done this, but I'd be interested to know if anyone's ever tried it.
