(Recognizing that this would never hold up in court as a legitimate argument) I would argue that there is no such thing in the first place as "ownership" of bitcoin. There is no real sense in which you can "have" bitcoin (aside from Casascius coins, but even then, those are just paper wallets made of metal). You never have anything. There are no "bitcoins" that really exist or represent anything in the real world. Rather, the blockchain keeps track of a bunch of puzzles and associated values, and you have the solutions to some of those puzzles, which (if you so desire) allow you to reassign them to new puzzles.

One could argue that if there are bitcoins associated with a puzzle to which you know the answer, you own those coins (in other words, the ability to spend defines ownership), but that gets messy too. What if multiple people have the ability to spend those coins? They could be locked in a multisig (escrow) or just have a script that multiple people can solve. If I randomly generate a key and find that people have sent money to that key (intended for someone else), do I own that money? (Of course, the probability of doing this is astronomically small, but it could happen.)

Of course, none of this actually addresses the legal questions you're bringing up.