Justification is one thing, but it may present some practical problems so long as they own large holdings in other countries. What if their creditors grabbed their real estate in London, for example?
My experience is that unless it is the specific borrower who defaults who also has assets overseas, then it is very difficult (as it should be)under US and international law to seize that property. If a Government defaults, it is hard to justify that you can seize some prince's real estate in London. Although given the interconnection between the Government purse and the royal family it would be a stronger case to make.