maybe some creative camoflaged external exhaust? it's filtered exhaust air, not blowing powder outside (only microscopic, not enough to see or accumulate), so i don't expect anyone would notice or have a problem with it.
here's the concept of a cyclone. basically, a high pressure blower is mounted to the top of a funnel. the funnel is mounted to a container (trash can, drum, etc.). an intake from the booth comes into the top of the funnel. the air is moving pretty fast coming into the funnel and at an angle so i swirls around the funnel. centrifugal force spins the dust/debris to the sides as the air continues down the funnel until it gets into the container. cleansed air has nowhere to go but up the center of the funnel, through the blower, and out the exhaust duct. there's a guy on ebay making and selling these (cyclone only) shipped for under $200. find yourself an appropriate blower and a container and you're pretty set.
average rule of thumb for flow is 100cfm per 1 square foot of open booth area. so if you have a walk-in booth that's 4ft wide x 7ft tall, supposedly you'd need 2800cfm. this is overkill as far as i've seen, not to mention the blower would be very expensive. we're planning a new booth that will be 5x7, and i think a legitimate 1600cfm will suffice.