According to the way this was answered in a post a few months ago, you can basically run through the fence and catch the ball. My feeling is that a temporary fence should be treated just like a permanent fence - with the obvious exception that it is succeptible to being knocked down or moved. In that case, it should be a judgement call by the umpires that if the fence was moved BEFORE the ball was caught, it's not a catch, because the fence was altered. A fixed, permanent fence may even be climbed to make a catch - provided someone is athletic enough to do that! IMO, standing on a knocked-down temporary fence section and THEN making a catch should not qualify as a caught ball.
There is, of course, safety considerations, but this is pretty much a common sense issue. You shouldn't be allowed to alter the playing field (move the fence) to make a play that otherwise would not have been possible!