Olivier Beaute had a fun quiz on this DB a while ago and asked what the main PVC pipe was on the boat - it was the cockpit drains! They are not GRP fiberglass covered except where they are tabbed into the hull. I've drilled through mine up high for A/C and Refrigeration cooling water discharges when I added those systems to my SN - they are just PVC.
As to the PVC pipe schedule, I think plain old white hardware store Schedule 40 rated at 220 psi should be fine - there is simply no positive pressure at all in the manifold (maybe a slight negative pressure, actually, as pumps suck water). That appears to be what Tom Cunliffe has used in the article I posted a couple of days ago. But, hey, overkill never hurt, either.
If I'm not mistaken, all Amels have factory installed fire suppression systems in the engine rooms.
Cheers, Craig - SN68 Sangaris, Tropic Isle Harbor, FL