Brian:
You're right. It has to be more than one pass, but there's no need for a pass for each bullet.
Duplicationg the particles, and switching to hardware-rendered particles, and then add a grayscale ramp to the rgbPP would make a perfect 2.nd transparency mask for compositting it. This mask should be used in adittion to the Alpha-channel in the bullet-trail pass (all the other geometry is switched off, only enabled in refraction and reflection, so you get a black scene render with only the bullet-trail visible, but the refraction and reflection of the scene should still be visible in the bullet-trail.