I have flown multiple times inside a large horse show arena...as a (free) favor to the owner (a friend) WHILE populated with people, many of them ON horses. The flights and video have been AT his request. He uses the videos internally for promotion of events.
Here is a short video. What you CAN'T see is the whole audience exactly UNDER the copter, assembled for the awards ceremony. I was about 20 feet up, clearance over the heads by about 12 feet.
http://flyfishingfotography.smugmug.com ... &k=PZBpxs5
The beauty of it, from a "horse-people" point of view, is that the horses were fascinated with the Phantom and virtually every one of them was watching in rapt attention, ears erect and forward....which makes for good footage.
We discussed this issue at length. Since I was acting as his agent, at his request, without reimbursement, we assumed (without contacting a lawyer) that any issues would be covered by his extensive event insurance, and he indemnified ME from individual liability.
I think my answer to you is 1)...these things are inherently unstable and unpredictable and in a public event liability for injury would be potentially extensive. 2) If you are acting as an individual agent for your own purposes...then YOU are at risk. 3) I think there are special situations in which you would be covered by the events/arena insurance...Protecting YOU from individual liability, but it is up to YOU to ask for and clarify that. And, of course, 4) You can't be paid for it (if you are in the US). The legal detail that the videotaping is (or may be) basically illegal (and should not have been taking place) might complicate how the EVENT/SITE insurance company might view having to pay for damages...even if you were acting as an agent for the event.
Your boom and alternate machinery itself presents some risks...to someone. What is your insurance for THAT?? (I don't need to know...just asking to get you thinking about THAT.