couldn't you use some co2 or presurized air... think lower power potato gun to launch a toiletpaper roll filled with BBs and some low powered firework like a mini airbomb?
Light the fuse, drop it in the pipe, Co2 shoots it out like a mortar, time the charge correctly and you have a paper tube falling from the sky that blows up at some point, either on the ground or in descent. It wouldn't be any different than an airsoft grenade going off in proximity of someone.
Of course, you could have a jam in the pipe, so you'd have to find a pipe strong enough to withstand the expansion force of your pyro going off. But considering that your mortar is only made of a toilet paper roll and masking tape, it's not going to contain the explosion and it will just blow out the end of the pipe.