http://www.gliding.co.uk/bgainfo/saf...winter2002.pdf
(Page 2, right hand side).
There's a lot of very useful work published by Tony Segal, although very little I think on the net. His area of research has been seat and cockpit design, and harness design - primarily for gliders (he's a retired GP and keen glider pilot who has spent the years since retirement doing safety research that he felt needed doing - if he's not been given any kind of medal for this, it's overdue).
I've personally used Tony's work as reference in the seat design of quite a number of light aircraft -
VP959 will certainly vouch for the validity of Tony's work, since he walked away from an aeroplane he had a too-close encounter with a crash of - thanks in part to my insisting on certain of TS' design recommendations in that cockpit.
Now this isn't directly related to vibration absorption, but nonetheless if you are interested in seat/harness design and avoidance of back problems, he's without doubt your man.
There was also a "spinal injuries in light aviation" conference about half a dozen years ago organised by the BHPA (British Hang-Gliding and Paragliding Association). You might be able to get hold of a set of the published papers from there by contacting the BHPA HQ in Leicester. It was a fascinating day, with a lot of papers on every aspect of how flying machines interact with aircrews spines, and how to make this as painless as possible. If no joy there and you're doing some serious research, let me know and you'd be welcome to borrow my set so long as I get them back.
G