Well, there is a margin, but not much. Not much at all...
Since it is a teetering rotorhead, it will need positive G during the whole manouvre, so one (not me!) could pull 0.5g over the top so not to loose too much height. The problem with this is that the responsiveness in the helicopter deteriorates, since it need the helicopters weight to pull on the rotordisc to respond. Less weight, less response - 0g no response, at least to my knowledge. There's been a 206 doing a barrel-roll due to stupid parachutists, having the same rotor construction principle and limitation.
The dynamics of aerobatic flight is very interesting and a good discussion would definately bring up a few learning-points on the design of a teetering rotorhead and other designs. And an understanding of why the POH says no to such things. Sometimes it's a design limitation other times a rule-limitation.
Saying that a barrel-roll is a 1g manouvre, shows a lack of understanding of the manouvre. First you have the pull-up, the more you pull it into the vertical, the more g you can pull over the top and keeping it positive. The interesting part is that if you pull it up too much, there's no energy or speed left for you to work with over the top. If you on the other hand don't pull it up far enough, you will probably see the ASI-needle pass the redline twice, and having to pull lots and lots of g's at the bottom.
From a teetering rotorhead point of view, I'd guess a split-S with a low speed entry would be better, but then, if you survive the first part, the obvious risk of overspeed is another thing to contemplate.
This is all in theory. Myself after competing on international level aerobatics in planks, I've never rolled a Cessna or Piper. I know they can do it, but they aren't built or certified to do it, so I don't. Horses for courses.
Bottom line is that if you have the experience and skill to perform it flawlessly, you also have the experience to never attempt it. If you don't have the experience to make it, it WILL be the last experience. And insurance premiums rises for everybody else.
Now get me that BO-105!
/2beers