If the contents are "sage", don't bother creating an actual link. Seems people don't bother to mouse over and check what the link is when the vast majority of the time it leads to "mailto:sage".
Of course, you might want to come up with another indicator that the post was saged.
the only reason for having a link field at all is that it's the traditional place to put "sage". a "don't bump thread" checkbox could do the same thing and be less confusing to people who are new to this kind of board, but we're a bunch of elitists who favor tradition over functionality.