I wouldn't personally know but i would assume that many developers pass on lines to mates if they feel they will never get round to them. Also think many of the closed projects get forgotten about as in lost inspiration. I guess as RodW said just asking (that is if you believe you have the ability to do it in the first place) most people will be happy if they had their reasonable shot.
Part of the reason for starting this discussion was an article in the last vertical life mag about closed projects. It raised a point about guide book writes including the bolter/developer of routes to recognize there efforts. Does anyone think this kind of approach would have an effect on the number of closed projects out there?