Perhaps the "Notify me about updates to ..." feature could be tweaked for this... if this is the means customers use to request in-stock notices, making an invisible change on the listing would probably trigger it? (A guess, since I've never used the feature.)
If you want customers to use this method, you can change the text to explicitly include the "or notify me when in stock" message.
It would be best, though, to write a mod for the rest of the functions as in the osC contribution.