This is what I'm thinking, but I think the issue would be rigging it securely on the rooftop side. Then again, you would have your stand right in the middle of your light, but hey, say it's a cookie and call it a day, right? :P
If you could rig the speedrail with the stand below, you might as well just forget all this and just put the light on the stand. If it needs to be armed out further, you could also use this terrace to put up whatever diffusion/cutters you'd like.
Interested to see what you end up with though. I did something like this a while ago but with a much smaller open faced 2k lamp. We used your initial idea and it worked out fine. Our fence was basically bracing our tubing down though, since it was much closer to the ground. Too bad it doesn't look like you have that luxury here.