The rogue epic particle is a pretty odd case.
Rather than having the particle spawn at the tip and then move down along the edge of the weapon (doable), the particle stays still and follows an invisible animated piece of the model that does the dripping motion. It also seems to be hardcoded in some way -- I can't get it to do anything other than drip a lot the first time it's supposed to drip (a 0.4 second window after 1.5 seconds) and then stop altogether. I know in the past I made a tool to increase the frequency of the drip by editing the timing of the animation, but understandably nobody bothered with that.
Worth noting that the rogue epic has the lowest model id number of all the epics (IT140) so it was probably done first. The other epics I've mucked with don't seem to have any such restrictions (other than the ranger epic lightning not being a particle at all, presumably it's a texture animation). A few particle weapon models pre-exist the epics though, so it's not the very first one they ever made.
I realized what the last major field in the particle data does: it's an angle in degrees to make a directional/stream effect disperse in a cone shape. It's what makes soulfire's flames lick about -- an 18 degree cone. Here it is amped up to a
62.5 degree spray.
There are a few flag fields that don't have clear effects yet; either they are really subtle or they are conditional somehow (just as the cone angle only does anything for directional streams) so I haven't noticed the difference with or without them. There are also 5 unknown fields that don't seem to have any extant examples of weapon models using them with non-zero values, even in the luclin and PoP+ stuff. I haven't tried any of them -- I assume they are fields that were used in the ancient spell particle system, which the item particle system presumably piggybacked on top of.
If you have a particle weapon it should be really easy to play around yourself! For posterity these are the settings I used for the awesome shaman epic (they were proving too awesome for me to change, couldn't try other things...):
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