Bitshares derived from Protoshares caught on exchanges during snapshot are legal property of account holders. If you had Protoshares resting with an exchange, you are legally entitled to your dividends- regardless of their participation.
For substantial amounts this may be worth pursuing.
Actually I am going to disagree with you here based solely on the fact that the point of everything we are doing is post-contract society and thus there are no legal rights conferred on your deposits.. you lent them PTS and they owe you PTS but in a purely voluntary manner and not based upon threat of legal action.
This would be like lending someone your cow, they care for it and milk it while it is theirs, and then they return the cow to you 'on demand'. It would be like lending someone your APPL stock, while they own the stock they get the dividends (it isn't yours any more).
If you want an agreement that retains your right to the dividends then you had better have it in writing. Remember also, that no one is under any obligation to honor PTS or AGS unless they want market acceptance. So please do not attempt to convert PTS and resulting dividends into a legal right.
Bottom line advice for everyone... deal with people you trust, don't rely on legal contracts or laws to protect you. Instead rely on market forces and reputation to protect you. If that isn't a strong enough guarantee then I wouldn't do business with that individual or company.