I am pretty certain you can attack from the rebel base... since you can move from it as if it was the system. However, how would retreating work?
Say the defending imperials have no units or loyalty in adjacent systems. Would he be able to retreat to the rebel base's system? Because the rebel player would have attacked from the rebel base space, not the rebel base's system... But if you counted that as the same then would you have to tell the imperial player where you attacked from? Because that'd give it away... Lastly, if the rebels don't reveal where they attacked from, would you force the imperials to pick another system if they selected the rebel bases system?
Yep, plus the attacking Rebels can't retreat back to the base space (if unrevealed).
They could retreat to the system where the base is (revealed or unrevealed) subject to the normal retreat restrictions.
For example a potential misplay would be attacking from the unrevealed base and then not being able to retreat to the base system, because of units/loyalty in other adjacent systems but not the base system itself.
A quick follow up question in the same category: I assume the rebels can attack out of the hidden rebel base space into systems adjacent to the system in which the rebel base is hidden; just like movement. Of course giving some kind of information to where the base is hidden. Is this assumption correct?