It is certainly allowed here in Canada. As quoted from our version of CAP413:
6.16 Relay of a Distress Message
A distress message repeated by a station other than the station in distress shall transmit a signal comprising:
the signal "MAYDAY RELAY" (spoken three times);
the words "THIS IS";
the call sign of the station relaying the message (spoken three times);
the distress signal "MAYDAY" (once);
the particulars of the station in distress such as its location, nature of distress, number of persons on board, etc.
Example: MAYDAY RELAY, MAYDAY RELAY, MAYDAY RELAY
THIS IS
CESSNA NOVEMBER JULIETT INDIA
CESSNA NOVEMBER JULIETT INDIA
CESSNA NOVEMBER JULIETT INDIA
MAYDAY
PIPER FOXTROT X-RAY QUEBEC QUEBEC
STRUCK BY LIGHTNING
DITCHING AIRCRAFT
POSITION: 20 MILES EAST OF WINNIPEG
ALTITUDE: 1500 FEET
AIRSPEED: 125 KNOTS
HEADING: 270 DEGREES
ONE PERSON ON BOARD
PIPER FOXTROT X-RAY QUEBEC QUEBEC
Mayday Relay certainly makes sense in very remote locations where the distance between two stations may make the original Mayday call impossible to be heard by a controlling agency. It is not a phrase I would use over the eastern seaboard of the US, but it is something I would definately use over arctic Canada, Alaska, Africa, etc.
Surely the answer to this most unlikely scenario is to make a judgement at the time. Isn't that what you're there for?
As it goes for everything in aviation, but then you're left with the Monday morning Quarterbacks spending hours ripping apart a decision you had seconds to make - even though that was
not the OPs intent.
Isn't our industry just great! lol