I hesitate to poke my nose in to a procedural area in which my professional competence is open to question, but I happened to be browsing through Annex 10 to the ICAO Convention last night (as one does to combat insomnia) and found the following in Volume II, Chapter 5. It seems that the "medical" term has already been assigned.
5.3.3.4 Action by an aircraft used for medical transports
5.3.3.4.1 The use of the signal described in 5.3.3.4.2 shall indicate that the message which follows concerns a protected medical transport pursuant to the 1949 Geneva Conventions and Additional Protocols.
5.3.3.4.2 For the purpose of announcing and identifying aircraft used for medical transports, a transmission of the radiotelephony urgency signal PAN PAN, preferably spoken three times, and each word of the group pronounced as the French word “panne”, shall be followed by the radiotelephony signal for medical transports MAY-DEE-CAL, pronounced as in the French “médical”. The use of the signals described above indicates that the message which follows concerns a protected medical transport.
I suspect that this probably originates in the ITU Radio Regulations which also cover maritime rtf. Maybe the use of PAN PAN MEDICO could cause some real confusion here