Release: 3.1 I am afraid there is nothing that Micro Focus can offer above REBUILD.
REBUILD is a utility which will attempt to restore corrupted files. As you have found, the success of REBUILD is dependent on the corruption level of the file. If the file is badly corrupted then it will fail to recover the data.
This is why it is very important for Customers to take regular backups of their data files. If they require the ability to rollback/rollforward then they should also use the transaction processing features within Fileshare.
One thing they could do is to use the byte-stream I/O routines to read the file at the lowest level - they may be able to extract more data this way.