I could suggest two approaches:
To use only built-in features, I'd try duplicating the keyed footage, adjusting the anchor point on the duplicate, and adding some extreme horizontal scaling (but no vertical scaling). Use Fractal Noise with some short, wide blocky settings and some judicious masking to generate a matte to use with the scaled footage. Apply the inverse of that matte to the original footage.
Alternately, you might check out
Idustrial Revolution's ParticleMetrix or
Trapcode Form if you want to use your video as the basis for a particle system.
Walter Soyka
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