Let me be more precise. What isn't it wages for a hour(s) worked? Once again you actually have to show up and work your first day to collect a signing bonus. I have never seen a signing bonus that is paid if the guy shows up and says, "I am here were do I sign for my bonus-- oh thanks for letting me sign I quit mail me my check!" You have to actually perform some labor in order to get that bonus.
In all the years I have worked in this field this is the first time I think I have had anyone try and make the case this isn't just simple wages. I have never seen someone try and make the case this group is trying to make that you can exclude it as a fringe benefit.
To me it is W-2 wages if your plan uses any of the safe harbor definitions of compensation it has to be included.