a regualr helicopter can not loop becasue the geometry and design if the rotor control system does not have the travel to provide enough negative angle of attack to support sufficent lift during the air noe up attitude and inverted part of the manuver. Or the rotor blades them selves are not constructed to support the flight load opposite that of normal flight. However some high performance and most modern military attack rotorcraft can.
If during a barrel roll of a large transport/utility aircraft, 747, 737, 707, etc. or any aircraft for that matter, if the pilot maintains 1 g you will never know other than seeing the horizon rotating outside. And yes, a cup of liquid will not spill or slide off the surface it is placed on as long as the manuver is conducted smoothly.
fact, a roller coaster never reaches 0g. It may get close but never reaches 0. A minimum gravitational force is alwasy maintined to hold you in your seat for safety reasons.
