Asking them to sign a message with a known Bitcoin deposit address perhaps
If the court has access to the deposit logs in user accounts should be possible to get that done
Why would the court want to send money (or bitcoins) to someone without knowing his identity?
Yeah, this will never happen in Japan. You need to remember this is a country where a signature is valued less than a rubber stamp (yes, which anyone can order to be made), I can't see the authorities ever allowing identification to be on anything other than physical paper. Japan seems high tech from the outside but Govt offices & banks seriously live in the 70s/80s with regards to record keeping.
Despite Kobayashi's inferences I also think there will be difficulty in distributing BTC under the legal system unless a claimants BTC address is included on the claim form. Only in that case could I possibly see some hope for that. Otherwise we may well see an auction similar to Silk Road, and a yen or USD figure then attached to claimants BTC holdings as at the time of bankruptcy filing or Gox's final trade. (most likely bankruptcy filing).