From the MtGox claims website (
https://claims.mtgox.com/):
Please note that the period for filing proofs of rehabilitation claims in the Civil Rehabilitation Proceedings ended on October 22, 2018 (Japan time). Therefore, Users can no longer file proofs of rehabilitation claims via the System.
so unfortunately it seems that it's too late to file a claim...
It does sound like that but the fact that they precise "via the System" gave me hope that it might still be possible in person or via a lawyer, etc.