Index.dat (with eg system version information) is encrypted, propably you could somehow edit it but it's located on read-only partition (vs0) so even after edit you can't write it back.
And if you change it with a single invalid byte, it will be rejected by the operating system. So you must know Sony public and private keys to make a new valid "index.dat" file.
Not entirely correct. The index.dat file is encrypted and hashed, not signed. You could reencrypt and hash it after you change it. The bigger problem is writing it to the vs0: partition.