Reading through the responses and OP's poser, I obviously see that there are going to be so many legal issues and enquiry that will emanate from transactions if cryptocurrency becomes properly adopted and regulated.
I could imagine people pooling together to sue exchanges if this argument is viable.
On this curiosity. Legally, you can't sue or get remedy for what you didn't bargain for.
If the fork of a coin as to receiving alongside your mother coin or any other agreement thereto was not included either explicitly or impliedly in the contract agreement, the appellant is likely not to succeed in the suit.
This is just on the flip side if there were regulations.