Well, it isn't really that stressful an application; the lockup is between the barrel and the bolt. All the Magazine cap/nut does is hold the barrel and fore end to the receiver. And a short section of CPVC pipe will take thousands of pounds in compression. CPVC is tougher than PVC, and the gray color is easier to blend in. I bought a 20 gauge 11-87 straight grip stock set for my LT 20 1100 and had to make a small spacer to fit in front of the barrel ring on that, and the CPVC has held up fine. But, if you are worried about it you could use steel pipe or aluminum conduit.
I have never seen barrel rings on 870 barrels in but two locations, the standard one and the Special Field edition which used a shorter fore end and a reduced capacity magazine tube, so if they have come out with something different for longer magazines, it is news to me. One of their big advantages has been all the interchangeability of parts, and I wouldn't think they would want to get away from that, but who knows.