Unfortunately this is a Flash limitation – I guess, a way to protect users from unwillingly modifying their Timeline. Probably quite reasonable.
Programatically it is possible to make this happen by showing the hidden layers (and potentially hiding them again).
At this point I'm not fully convinced whether this is a good thing to have or not.
Would you, please, give me a scenario where an animator does not see the elements, but he (can be she) would like to modify them blindly.