<playing "ring around the rosie"...>
Unfortunately, we are limited to a flat plane emitter. Having said that, though...
You could set up a ring of emitters with a rotating anim all pointed down and in to fake what they're doing with ellipsoids.
I can't think of any way to give the *particles* a spiraling motion, but you can emit them from orbiting emitters and they would fall, move toward the annihilation point. As long as it was downward, they would accelerate.
<...with his raven>
