You should be able to use extra fields to do this.
You can read about setting extra fields in the manual. You can create an extra field to use to store the "Set by Mail" variable for each product.
(You will have to do a bit of a mod to get it to do a radial or pull down to set the "Y" or "N" - or you can just enter the "Y" into the default text-box that xcart sets up for you.)
There are several posts on the forum describing how to retrieve the extra fields into your templates. You could start with
this thread - it helps explain how to get the variable and do if/than based on the setting.
You can read the value of the extra fleld you make on the category page and the product page. You can do an if/than to display the icon when it is set to "Y"