[Sohcahtoa82]

What he's saying is possible, but it wouldn't be a barrel roll anymore.

You can experience 1G while banked 90 degrees by pulling up slightly. However, the plane will be losing altitude as the wings are not generating lift.

[pedrocr]

Once you are banked 90 degrees you can do that but how do you get to that bank while maintaining 1G pointing towards the floor at all times?

[falcolas]

By smoothly transitioning into the turn as you roll. The simple act of rolling the aircraft doesn't change the amount of lift being generated towards the roof of the aircraft. In fact, if you don't increase that force intentionally, you will turn and lose altitude.