The international standards for distress and urgency VHF calls are specified by the ITU for all signatory countries (pretty much all countries are signatories), and they have a provision for using the ship's call-sign .Note that use of DSC and the MMSI isn't too common worldwide yet, but it will someday be commonplace to use DSC for distress and urgency calls. DSC equipped radios have a log, and MAYDAY signals cannot be deleted from the log, and the radio beeps loudly until the message has been acknowledged so the excuse "I didn't hear that call" no longer applies - whether intentional or unintentional
I know it's the "done thing", I'm simply questioning it's practicality. I thought I saw it's OK to use the Reg# when I looked into this yesterday. Used far too much time responsing above, I'll look for it later.