I don't believe this is possible with X-Cart and I haven't seen any modifications yet that actually enables this specifically with the Wishlist.
However there are some modifications that comes very close to what you want I think, try take a look at the "Back in Stock Notification" mod here:
http://www.bcsengineering.com/store/back-in-stock-notification-x-cart-mod.html
Or even the "Preorder Backorder" mod here:
http://www.bcsengineering.com/store/preorder-backorder-mod-for-x-cart.html
Cheers!