Here is an easy way to install fire sprinklers using Aquasafe - Aquapex plumbing by Wirsbo. I was wondering about using plastic lines myself and came across this while researching details on the Aquapex system I used to plumb my cabin.
http://www.wirsbo.com/main.php?pm=1&mm=3&sm=1&pc=homeowner/ho_mm3sm1.php