There can't really be a Rudy of running, because it isn't a team sport in the same way. There are tons of good programs that allow "slow" guys to be on the team.
And to really be Rudy, this hypothetical runner would have to be given a chance on a huge stage, which would only happen when a team's position in a competition is secure. You obviously can't make substitutions in XC, and in track, you generally have actually qualify for the NCAA meet, so no "slow" people would be available. The only exception would be if a team at nationals, going into the 4x400, had its finishing place already locked up, and then the coach decided to throw in an alternate. But even if the team competition is locked up, the relay is an event in itself, so putting in an alternate would be like throwing the event unless there were really no alternative.