I'm doing something similar, and this information helped me to get one particle emitter behaving correctly. When I put a second copy of the particle emitter into the chor, I have not found a way to make the second copy of the particle emitter object's sprite emitter "rate" value change over time.
I tried doing what I thought I did the first time around, changing it in the shortcut to the particle material under the original particle emitter, not under the shortcut to the particle emitter in the chor (the latter shortcut doesn't have anywhere to change it). This changes the value, but does not create a key, and the new value is constant for the entire chor. I have clicked on the little lock that comes up when you change the value and I selected the clock icon (change value over time), but it does not make the value change over time. I've also tried changing the value under the Driver hierarchy, but this does not help either (under the driver there is no lock to select change value over time).
Of course I can create several separate particle emitter models and set each one up the same way, with only one of each in the chor, but I was curious if I was missing something that would allow multiple copies of the same emitter.
Also, I don't fully understand drivers. I looked in the tech reference and did not find much about how to create and use them, except as an artifact of other operations. I also searched the Tutorial forums and did not find anything that really goes into detail. Is there anyplace else I should be looking to dig deeper? Thanks!
-Vance