even if you can get into the cargo hold (the 747 you can go down into the eec bay via first class, then aft into the cargo) the way cargo is packed there would be no room to move about or open doors.
cargo isnt a little bit here and there that you can walk around it (like in the movies), its normally packed into the exact shape of the cargo hold, with no more than a few centimeter gap around it.
the 747 classic with the cargo container crew rest is a sealed unit, there is no way from the container into the hold.