I think the rules actually cover this, but I want to make sure other people are reading it the same way:
If the Imperial player sits an AT-ST on top of the 'Entrance' space during one of the missions that involve an escape, Rebel players can still 'Escape', because the definition of 'Moving Through a Space' (Rules Reference, pg 19) says that the figure enters the space but cannot end its movement there. So by the definition of 'Escaping' (Rules Reference, pg 12) they have spent a movement point while in a space adjacent to the Entrance token.
Does that seem right?