Rich,
This is all conjecture on my part, seeing as I've never fired a plaster projectile, but while I'd think it would be less dangerous than iron, or lead if it hit an object head (I think it would just crumble) on; if it hit something hard at an angle, wouldn't it possess the same odds of ricocheting off that object, and traveling on to possibly cause harm to someone, or damage something?
Rather than trying to find a safe projectile, I'll stick with the old firearm's safety rule: "Be sure of your target, and what's behind it."