to get the equivalent of a "registered letter" you might have to use an international carrier such as DHL and print out the final tracking/delivery at destination as a proof (in any law suit that print-out might need notarization) but I would address it to the legal dept of Peregrine -- individual recipients might just toss the mail
OR, your buddies might get a law office here to send the missives on via registered mail, receipt required.
I would also mark the King letter PERSONAL ATTENTION OF
best, N
PS Mail is mail, going into a corporation, Registered is proof it got to the destination, not necessarily to the individual AND a "served letter" you refer to it is not -- for instance, court judgment would be served by a peace officer, at my expense, to the individual owner of a business who neglected to respond to my registered letter demanding payment of damages that the court awarded to me. After that I could remove his business property or attach his bank account