Of course. If evolution is true then an event forbidden by evolution could not occur. But whether macro-evolution occurs or not is what we are considering. If there were T-Rex eggs, I would pretty much figure on no mammaries regardless of evolutionary theory.
Why?
There are platypus eggs. Should a platypus have mammary glands?
Hint: gore3000 says the platypus doesn't fit on the evolutionary tree. He's not the one to ask. It does.