This is the problem with Hotlink protection, it does not fair well with proxy users and the like. Perhaps your best option is to give up trying to prevent download but instead applying some copyright or other text notice within the video itself? Such as a line of text referring to your website at the top or bottom of the video screen.
At the end of the day, you could come up with some fantastic way of preventing download, but then again there is software which will copy any animation running on a desktop by way of screen capture. There is just no completely effective way of doing it.
