So you're saying that when you do a for each loop on a family of objects the loop starts with the object that has the highest (or lowest?) y position? That's handy to know if that's the case, but it's not well-documented and it's certainly not intuitively obvious.
I think a nice way to handle this would be to have an option to automatically z sort items by y-coordinate. Linkman's suggestion would give more flexibility but I think the iso game z-sorting issue is the main motivator here and this auto sort would work well for that problem. Also, maybe you could auto z-sort just the objects involved in the collision. So on (for example) a busy isometric rts layout you'll save a lot of unnecessary z-sorting.