Can pilots use PAN PAN Medical, even if they were not on MedEvac mission?
The UK CAA Radiotelephony Manual (CAP 413) has the following to say about it:


  PAN PAN MEDICAL is not used in the UK but refers to a Geneva
  Convention term associated with medical transportation by parties
  involved in a conflict.
  
  Use of this term is not appropriate in the UK.


Which seems to be a pretty definitive no. Personally, I'd not heard of the term but assuming the CAA's interpretation of the ICAO's definition (i.e., for use during war) then I can't imagine it ever being appropriate outside of that use.

That said, why would you need to? Using a normal MAYDAY or PAN call and describing the emergency as a medical one is perfectly acceptable and would be fully understood by all parties.