This method is negated by any wallet stealer since both the thief and victim would have the private key. Given that info, how do you prove that you were ever the lawful owner of the BTC at that time? Only thing I can think of is maybe a receipt of some kind (exchange logs?) and that doesn't take into account the fact that the BTC could have been traded willingly off network by sending the private key to the buyer (or thief [I can know as a 3rd party{you can't either}]).
As I mentioned in my blog which started this discussion, proving original ownership of the privacy key will be difficult and certainly will be subject to a finding of fact in any litigation.