Since you are a lawyer why don't you define legal possession and how that concept can be applied to bitcoin for starters.
It is certainly subject to debate. My argument would be that possession of the privacy key constitutes the ownership and right to access the value denoted in the blockchain ledger. That is the basic premise of Bitcoin, correct?
Really? So if I post my private key here then everyone that sees it on this forum owns the btc in that address? What about any btc that used to be there? Do they somehow become past owners of that as well?
Actually deliberately posting it here would be an implicit invitation to add it to a wallet just to see what's in it. For example, the "correct horse battery staple" brainwallet passphrase used as an example. (Don't actually add it to a wallet because it is so constantly flooded with transactions that it can easily crash your client.)
Posting a private key with funds in it is basically yelling "Hey here's some money, and I'm leaving it here while I go away!" That may make the inevitable thief still guilty of something, but good luck getting any sympathy from a jury.
If, however, someone obtained the private key illegally, such as with a keylogger or by finding the Post-It note where you left it under your desk, even if that is almost equally stupid, obviously they don't possess the BTC. I assume OP meant by "possession" what is often meant legally by the term, that is, legitimate possession. (Even a thief, though, is actually in "possession" of a stolen item and a drug dealer "possesses" illegal drugs even though the law recognizes no ownership right in such items.)