Had the same problem. Crudely lashed the top tether belt around the rear
headrest using the intended tensioning loop. If you think about it as an
engineer might, instead of a product liability insurance underwriter might,
I think the westy luggage rack cleat would do it.

http://www.gowesty.com/Merchant2/merchant.mvc?Screen=PROD&Product_Code=GVW-231-067&Category_Code=

If 2 X 6mm bolts wont hold it, you have a MUCH bigger problem resulting from
a crash than the top of the seat coming loose. I would be suprised to hear
that the breaking strength of the ~1-1.5" (sorry for the english units) is
more than 300#.

This is the solution I mean to apply but there are several other things
ahead of it in the cue.

Robert
90 MV