I guess what I'd do would be find an appropriate cyclone, and then make a nozzle that adapts to the fire extinguisher that you then force air through to "fluidize" the fire extinguisher media. That's how the stuff is designed to work, the air you pump into the thing will help the material flow out. Optimally this nozzle you design will also contain an outlet for the fluidized extinguisher media to go.
I'm going to venture a guess you shouldn't need too much air flowing into there to help the powder want to come out. The filter will likely be necessary though, otherwise you'll get a big cloud. You might try it on one or two with very low air pressure, and tipping it into garbage bags or something, might get lucky and not make too big of a cloud, if you use super low air pressure to fluidize.
You might want to consider some PPE if you haven't already, such as HEPA respirator cartridge, just so you don't end up breathing too much of it. Whether it's necessarily harmful or not, a bunch of fine dust in the lungs probably isn't the best thing to have.
