Well I've had a bit more of a play and, in all honesty, it's a bit tricky to dream up a more appropriate experimental setup that actually does anything different to what I first did.
I tried fixing the spar at both ends then pulling the centre down - I'd need more than 240fps to see anything as even the wobbly 3PT stops in under two seconds.
I tried loosely damping one end - pretty much as above.
I even tried adding an end weight - definitely
I did come up with an improved way to record the experiment but all in all it's the same result. If
the perceived advantage of Aerostuff spars does indeed come from superior vibration damping then it's not by a huge amount and the only marginally heavier yet far cheaper (under a tenner each) Avia G-Force SULs should be an even better bet for a standard kite.
Not sure where this leaves us.