Thanks for the suggestions,
@fabs, but I did try loading from the windows directory of the original drive and got the blank ProduKey window of keys found. Also tried both 32 and 64-bit versions. Regbackup hives have zero length, unfortunately. I considered importing the SOFTWARE hive with regedit to see if the required key is intact or would be found by ProduKey but chickened out when it said it would load them to Computer and delete existing keys.
The client has suggested I discontinue the search; he finds that Ashampoo Office Free is good enough. Many thanks to everyone for the helpful suggestions.