Rather than emitting particles, might an easier solution be to have another dividing line in your pixel shader?

So everything below the first line renders the geometry solid, everything below the second line renders your sparkly reveal effect, and everything above the second line is clipped.

It might be tricky to make a nice reveal effect in this context, but I think it'll be a hell of a lot easier than generating actual particle geometry.

another possibility:

a simple particle system (say 20 animated 3d billboards) that you simply move up the model with your reveal. make it busy enough and it should look fine.

maybe use one for each leg, then switch to a single 50 particle system for the torso, etc.

I'd second Columbo. With a clear dividing line and a pixel shader than already does the clipping, applying a "sparky" texture might be enough. In general, it does not look like this effect is going to take more than a small part of the screen so it needs to be fast. Particles don't have this property, at least for the blending involved.

Particles don't have this property, at least for the blending involved.

quite true. depending on the exact effect (a sparkling texture vs a cloud of sparks) a pixel shader will be faster. particles wouldn't really be required unless the "sparkling" was supposed to occur beyond the boundaries of the skinned mesh.