I don't believe that in most sane juridictions a court would keep a suspect in prison indefinitely when the suspect claims to have forgotten the passphrase for a bitcoin wallet. I think the court would have to find the suspect guilty and sentence them to an appropriate custodial sentence (which in my personal opinion would be no longer than what they'd get for deliberately forgetting the passphrase).

The only incentive for the suspect to remember the passphrase after being sentenced would be earlier parole.

If the suspect tried to use the bitcoins after being released then another charge could perhaps be brought for perjury.

None of this is specific to bitcoins or totally new. It's roughly the same situation as when the court thinks that the accused knows where the stolen money is hidden, but the accused claims not to know. Courts must have dealt with hundreds of cases like that, so we could look up somewhere how they handle it in different jurisdictions, if we really wanted to know. I'm curious about it, but perhaps not that curious.