modpost is not supposed to be used outside the kernel build.
And therefore if we introduce a new option then the infrastructure
to enable that option should also be in place.
In this particular case I cannot see why we should add the possibility
to include local symbols, in other words do not add the option.
Wait a few days before you kill it, maybe others see the usefulness of it.
I checked if there were any options supported by modpost that
was not configurable in makefile.modpost.
And I could see that the -M and -K options in getopt() was leftovers.
The code that used these option was was dropped in:
a8773769d1a1e08d0ca15f890515401ab3860637 ("Kbuild: clear marker out of modpost")
Could you add a patch that delete these on top of what you already have.