And to add, a barrel roll is not a +1G manoeuvre, it is a positive G manoeuvre. You have to have more than 1 G in the initial pull up and you have to have more than 1G during the pull out at the bottom. How much depends on exactly what others are speaking about here.
Yes it can be done in almost any aircraft, but if you do it in one that is not an aerobatic aircraft then you are a test pilot and I hope you have test pilot skills in order to ensure you do not stress the aircraft.
I reckon the barrel roll has to be one of, if not the most, misunderstood aerobatic manoeuvre out there. People who say it is only +1G have never flown one.
To do one at constant pitch and roll rates requires real skill. You will notice in the videos that Mr Hoover doesn't do a textbook barrel roll (and neither is his loops) they are modified to achieve the desired wow factor, whilst maintaining an acceptable degree of safety close to the ground (this by the way is what made Mr Hoover such a good air show and display pilot - he knew how to achieve the wow factor whilst still maintaining his own level of safety). Notice how he pulls up to between 45 and 60 degrees nose up before he begins his rolls, this means he will have a nice high nose attitude before he begins the rolls (feet on the horizon at least) but more importantly it means he will have very little if any height loss and it gives him plenty of room to adjust his pitch rate coming out of the manoeuvre to ensure he doesn't overstress the airframe but also have the pull up completed close to the ground for the extra wow factor. The same applies to the loops (most air show pilots do it), the loops aren't round, they are elliptical with the top of the loop abbreviated ie the pitch rate is increased so that during the pullout again you do not have to load the aircraft up to ensure you lose no height and can then adjust your rate of pitch to have the loop finishing close to the ground for the wow factor.
Anyway back to the barrel roll and topic at hand, it is most definitely an aerobatic manoeuvre, I believe the author was simply trying to state that it can be used to get out of an unusual attitude and was trying to get his audience to be less nervous about it by saying it is not an aerobatic manoeuvre. The latter is obviously wrong.
Cheers
CB