hmm guess I would go w/ sullivan then since I can't remember where I heard the accident story. There are other memorial plaques right there for other climbing parties that have fallen off the route too.
I think it is a memorial to a climbing accident way back. Remember, Cooper Spur was the original "standard" route on Hood long before Timberline or T-line road were around. At one point a steel cable was hauled up the route for people to use as a hand line.
Now that's a good one. Did you come up with that on your own, iain?
It's on the back of the helmet of someone in Portland Mountain Rescue, not sure who said it but pretty funny.