There were even 3 GS winners out of top 8.For instance, in 2002, Thomas Johansson and Albert Costa were not in the top 8 of the race whereas they were both slam winner. Only one of them could go to the YEC, Costa !
It would make sense to me that any other Grand Slam champions (i.e. Johansson) were offered the alternate place first, but I don't know whether that's the rule or not. I only know that Henman said he wouldn't travel as an alternate that year and Sampras didn't play any events after his US Open win, not sure about Roddick, Haas or Nalbandian though...There were even 3 GS winners out of top 8.
ToJo was the first alternate there, but does any one know if it was thanks to the rule (2nd GS winner out of top 8 is first alternate) or because Henman, Roddick, Haas, Nalbandian and Sampras did not want to come ?