If you don't need effects on target, we long used Nerf rockets for airsoft mortars. The ones that look like a miniature versions of the football with a long tail and fins. They'd even just fit into a 40 mm, so are the first single big projectiles that we (anyone in airsoft?) fired, using empty but gassed BB showers to shove them.

I'm not sure they make these rockets anymore but there are plenty of other Nerf projectiles that are designed to be fired at people so I tend to believe are as safe. Shouldn't even be very hard to test but especially when you are talking about indirect fire, only the firing crew is right next to it, when it's coming down at the target it's at the low terminal velocity for a very lightweight draggy item.

(Funny stuff to look out for as you start this: the total lack of mass for safety means that it's pushed around by wind really easily. When you're talking indirect fire, that means you're shooting it way up in the air and very often there are different winds up there. I've lunch stuff in relatively still air and watched it hit wind 30 ft up and turn, come straight back at us 😮)