I cannot remember seeing a new IFA come up after the end of the minor league season. And, since this question comes up all the time, I've been paying attention to this for about 3-4 seasons of 3 teams, and I can say for sure I have not seen one in those 10 or so team-seasons.
+1 on Mike's comment about being able to negotiate with them at least until the WS (I've never tried later than that).