I flew off the top of a wheeled tree trimming cart left on the baseball outfield next to the Field House at WSC (now WSU, aka "WAZZU") in Pullman, WA. My brother started the engine and launched the plane (steel finned Fox .15 profile "Green Monster") after I climbed up the ladder with the handle. Could do outsides from level flight, but I knew that loose lines would be pretty ugly, with no place to run. The landing was interesting, but no harm done.
Local lore has it that Jeff Rein flew a fast combat model off the wooden "Wilburton" railway trestle in Bellevue, WA. "Wing Unders" would have about 40' ground clearance on 60' lines. I wish I'd been there to witness it. Maybe Kenny B, Danny Dirt, or Howard were present? Jeff has won "Bladder Grabber", NW Combat Champion, was the guy that invented "line sliders" for Navy Carrier, and recorded the hit song "Ballad of the Folken A". Flies stunt pretty well, too!
http://en.wikipedia.org/wiki/Wilburton_Trestle 
Steve