Jimbo has given me his blessing, so I'm proud to bring to you his very handy script for slowing down, speeding up, and completely stopping particle systems, as seen in vids such as Bad Medicine.

https://raw.github.com/jimbomcb/SFM_Rigs/master/setup_particle_timescale.py

Put that in the standard folder and there you go. Attach it as a rig like you would do to any other model.

Be careful: it might screw up your project. Always make backups.