Unfortunately, we have no method to customize that message. However, you could add a line of custom text/HTML to your cart, at least on pages where you already know items are out of stock, by adding the following lines to your View Cart code on each page where you want the custom message to appear, just before the "// -->" line in the standard View Cart code you obtained from Seller Admin:
function EJEJC_config() {
EJEJC_POSTCALL=true;
}
function EJEJC_shown() {
jQuery("#tdSell").attr("innerHTML","If your item is out of stock, please email xxx@xxx.com to receive notification when the item is back in stock.");
}
If you're already using cart customization, just add the jQuery("#tdSell").attr("innerHTML","...etc."); line to your existing function EJEJC_shown(){} section.