Perhaps the legal response here should be dismissal.
It may technically have similar properties as bearer assets, but legally it's different. A bearer instrument is a document which is an explicit legal contract granting certain legal rights. A Bitcoin private key is not a document or a contract (an implicit contract as in "the system works this way, you can look at the description and everybody should know that" doesn't count), it's just some numbers, so it does not fulfil the criteria of laws about bearer assets. And if it did match these criteria, that would not be a good thing, since there are all kinds of prohibitions and restrictions on bearer assets due to their potential usage in money laundering. So while it is very similar to bearer assets regarding de facto control, it's not similar to bearer assets regarding de jure ownership.
Has this been tested in court? Lawyers say all kinds of things, but the set of proven-correct things they say is smaller than the set of all things they say.