I understand what you are saying. But.... Why couldn't you make a mask in Aftereffects (or shake, Make it have a large Feather, and then add it to the frames (rgb) that you export from Maya, So, that...

would you be able to just ad a mask in after effects, (with a feather) to make it look like it fades away? But, that would require more than one pass, an edditional pass for each bullet trail.

Hey, Well here is my Particle emitter, particle trail w/expression, and a test bullet. If you get it to work any better or make any changes please let me know I would love to have a copy of it....