No, you would have to export your library, add the modified labels and then import again. Not elegant, I agree.
The problem I see, is that this style is very specific: 3 first of 1st author letters with and without a, when is the a introduced, only when a b is present? or would there be a MEY09, MEY09a, etc- what happens if the authors last name has only 2 letters?
And has a number of variants that I found on with some googling – 4 first letters of 1st author; the first letter of the first 3 or 4 authors, Which complicates matters.
And would the a,b,c you suggested be hardwired or is it publication specific? Is this a specific fix, you are suggesting or is this a published requirement?
All of this must be taken into account before the company undertakes the modification of the database programming and macros development within the program. A need so specific might be solved by the organization requiring the style? I think that is what developers have done here with merlin which I don’t pretend to understand.
good luck!