Anyone got a solution for the bullet-trail not fading away?

In the original (The Matrix), the bullet trails fade away, and seems to dissipate (is that the right word??) into thin air... Just vanishing, wich of course is natural (???), because the shockwave would loose it's energy.

Still I can imagine just one way to acheive this (in just one render pass) and that would be setting the refraction and reflection index on a PerParticle basic.
The only way I can see that happen, is having... like 73 different materials, and make a script that changes the material on just a single particle, like you have "Matrix-Shader001" to "MatrixShade073" set up with decreasing/increasing refraction and reflection index and make a script or something that uses the agePP to change the particle material/shader.
-Code-sample that would work in VisualBawsic:
[Particle(index).material=MatrixShader & particle(index).age]
If the .age attribute is 16, this would make the particle(X) receive the shader/material "MatrixShader016", wich have a lower refraction and reflection index than "MatrixShader015".

I don't know too much about MEL and such but it would work in Visual Basic


Another method would be rendering just the bullet-trails, in one pass, then make a copy of the particles, and aply a gradient map (rgbPP), and hardware-render them, as the second pass, and then, rendering the scene, and composite them toghrether using the hardware-rendered gradient (rgbPP) as the alpha-channel for mixing the bullet-trail and the "beauty-pass" togheter.
This would make the bullet-trail fade away, but not as nice as adjusting the refraction index individually (based on agePP) on each particle.


Any coments on the previously mentioned methodes, or MEL scripts that could work in the same fashion as my VB-thingy would be aprechiated
-DX-