I am skeptical you can do significantly better.
You could add an indirection layer between index name and leaf directory but this is adding another concept on an area that is already very crowded:

• indexBase: used to share the same root dir for all indexes
• sharding that requires subdirectories
• source vs destination directories in case of master slave

Adding another property seems wrong.

One way out might be to use a template property to build it from the base, the shard if, the index name. There is another JIRA AFAIR