Alright lets just bug out and call it even. How about this. There was a theory of a gas that could morph humans into aliens. Lets just say when ripley whacked the queen upside the head with the loader the Queen had a bit of flatulence and farted out said gas. I mean come on, she is a weapon, thus her farts must also be weaponized. She was pretty big and it's not impossible that the gaseous concoction floated around for a bit before getting stuck to the ceiling somewhere else on the sulaco. Thus explaining how an egg could have magically appeared.
I mean come on, if the space jockey are as perverted as Ridley is making them out to be then they surely must have thought about this. Maybe the egg sack was just to control her flatulence so she could willfully place the eggs where she wanted them.