Having rolled, split S'd and looped several helicopter types, and also helped lay out Comanche's rotors for the air combat mission it accepts, here are some thoughts:
1) The barrel roll is made with a pitch up followed by lots of lateral cyclic. The aircraft scribes a spiral flight pah and holds positive G. This is the common helo roll, albeit with some speed loss as Crab suggests.
2) A rapid lateral stick input will produce zero or negative G at the 180 point, and is very much like the aileron roll in an airplane.
3) These maneuvers do not tax the tail cone or tail plyon at all, those components are designed for very high yaw thrusts, beyond anything experienced in a roll. The tail rotor blades, hub and shafting, however, are subject to lots of flapping and precession forces, and can achieve limit loads at high roll rates, maybe 100 degrees per second. Fantails like Comanche see no such forces, and so do not inhibit rolls at all.
4) Loss of control is a big factor in most helo designs due to the issues of rotor control loss at zero G. These maneuvers are quite likely to cause rotor contact at the hub, and on the fuselage, that will be catastrophic. Teetering rotors are verboten in this discussion, and low offset articulated systems are marginal.
5) The barrel roll is a semi-useless maneuver, basically for air combat when a rapid slowdown is needed (the spiral flight path is much longer, so excess speed can be burned off while tail chasing a bogie), the aileron roll is used to rapidly change the direction of the lift vector as a climb/descent or turn entry maneuver.
6) Helicopters in air combat do very different things, and they are quite a bit more maneuverable than airplanes, just not in the airshow sense. Most helo air combat will involve rapid turns and then weapons engagements, and little vertical maneuvering. Classic WWII/Vietnam airplane air combat involved jockying around to align the fixed guns with the bogie, a style of combat that is now basically obsolete. All aspect weapons and supermaneuverable aircraft (F-22, Comanche) will make snap turns and rapid shoots much more the style.