OOC: That sounds like about as close to consensus as we're likely to get. So here goes: Get everyone down to ground level, near the opening of one alley, ready to run. We grab a pile of spare laundry from one of the lines, and dump it right inside the mouth of another alley- Rikka can stay on the roof above that alley, out of LoS from the square and guards. Once we're ready, she can fire off a Chill Wind burst, targeting the laundry pile. On a hit, that includes a Slide 1- so a pile of stray cloth would sweep out of the alley, make a swirling cloud and collapse. I'd say that prompts a closer look, but no obvious alarm- not even a shred of human visible. If they take the bait, everybody make a run for the door- as noted, 10 squares is a double run for everybody.
As noted, if it doesn't work, we haven't really revealed ourselves, but I don't want us to get too bogged down in our brainstorming.