After a few tries, I got one mtoa.mod file that works for multiple Maya versions:

+ MAYAVERSION:2015 mtoa any C:\solidangle\mtoadeploy\2015
PATH +:= bin
+ MAYAVERSION:2014 mtoa any C:\solidangle\mtoadeploy\2014
PATH +:= bin
+ MAYAVERSION:2013 mtoa any C:\solidangle\mtoadeploy\2013
PATH +:= bin

I had to repeat the PATH line for each module, otherwise the bin folder wasn’t added to the PATH.

I just had to set MAYA_MODULE_PATH to point to this mtoa.mod file, and then I could load MtoA in Maya 2013, 2014, and 2015.

This could be useful if you have multiple users sharing a single machine, because with this mtoa.mod, you don’t have to worry about putting version-specific mtoa.mod files in every user’s Maya folder (by default, the MtoA installer puts mtoa.mod in the user’s $MAYA_APP_DIR\\modules\\<version> folders).

Another way to do it would to be put the mtoa.mod files in the default shared modules folders. For example, on Windows:

C:\Program Files\Common Files\Autodesk Shared\Modules\Maya\2013
C:\Program Files\Common Files\Autodesk Shared\Modules\Maya\2014
C:\Program Files\Common Files\Autodesk Shared\Modules\Maya\2015

On Linux:

/usr/autodesk/modules/maya/<version>

On Mac OS X:

/Users/Shared/Autodesk/maya/<version> for MacOS