Which I am sure your aware is against the tos for use of technet keys. Those keys are for testing purposes only, or for installing the OS on your machines at work. They are NOT to be used on clients machines.
With vista and 7, as long as you use the coa on the sticker, the technet iso will install and activate. If the coa is unreadable, and there is no recovery media / partition, the only way you can reinstall windows is to contact the manufacturer and order some recovery cd's, or purchase a new windows cd and coa. Obviously the recovery media route is less expensive.
It is not your fault the clients coa is unreadable, sometimes its not the clients fault either. The coas on vista machines, I have found in the past to be made of paper, and the code easily rubs off. Which is one reason why, I always make a note of the coa, and store that in my clients database. Also I put a piece of selotape over the coa to stop the code from being erased.