Any way to make Particle Emission Count be a Driven Key?

So far, most any parameter I’ve tried to make a driven key will let me do it. One thing I do is to set up a single armature that can drive multiple objects rotation/scaling/etc.

I cannot figure a way to do the same thing to a Particle Emission Count to adjust the total amount of particles. (it doesn’t let you Add Driver)
This would be really nice to use this concept to control the “spigot” for a bunch of objects with particles systems…

any good/nifty ideas on how to do this?