Another idea as to how to display a negative value is have the bar be empty at zero, but turn red at any negative number. Once a creature is dead, the X status is used, so the red bar would just be confirmation and wouldn't overlap with other tokens' bars. That way, if the bar is red, but there's no X status, the creature is still alive, just not dead yet (Pathfinder and DnD type rules, dunno if that helps for other games or not).
I find I have to make sure my dead tokens are at zero, because the field can get somewhat cluttered sometimes with overlapping bars.