Aah - thanks PU. Missed that particular bit of subtle qualification! I agree that the event isn't consisting wholly or principally of the flight, which means the 500' rule stands. I was unsure of my interpretation, since I've never heard of anyone else mentioning an exemption when talking about things like this...
Sycamore - I will give David Evans a call anyway. I'm not planning to be within 500' of anything anyway as it happens, due to noise and obstacles in the vicinity so I won't need an exemption, but it's interesting to know that one could be offered in theory - I thought it was a hard-and-fast rule.
Thanks for your help!