In glinks 3.3 we added the ability to specify columns or if that column was empty, an alternative:
Added new syntax to category and detailed url formats. This additional syntax allows you to use a format such as: '%Category_URL|Full_Name%', where Category_URL is a new column in the Category table. If Category_URL has a non-zero length value, then it will be used, otherwise Full_Name will be used. You can have an unlimited list of columns to try (the first column to return a value will be used).
Not exactly what you need as you can't specify a string default.