As you already suggested, the
ObjectTemplate.PositionalSpeedInDof CRD_NONE/10/0/0
only works for real particles, not for PCOs.
Basically, you are having the some problem we had some months ago when we tried to understand and perfect "ghost aircrafts". Empty PCOs that nevertheless fly straight ahead. That's why I guess there would also be hidden the answer to your problem.
Try attaching an engine to your PCO, set input to Rocket type (continuous), give it ultra high perfomance (acceleration and speed). That
should make it fly straight ahead in direction of fire (DOF), but you'll perhaps still have to tinker with gravity and stuff, making sure it really does fly straight.
If you need further info on adding that engine, most of it should be explained here:
viewtopic.php?f=3&t=4213We refined that pretty much IMO
.