I'd like to add to pkgsrc/mk/bsd.pkg.defaults.mk:

MANDIR?=       ${LOCALBASE}/man
# Directory where manual pages and man directories are installed.
# Possible: any path you like within ${LOCALBASE},
# like ${LOCALBASE}/share/man.
# Default: /usr/pkg/man

(I say within LOCALBASE to make it easier.)

Then slowly convert all packages to honor MANDIR.

For GNU_CONFIGURE-using packages add to bsd.pkg.mk:
CONFIGURE_ARGS+=	--mandir=${MANDIR}

Other packages have patches that hard-code ${PREFIX}/man which can be
easily fixed.

Many packages use INSTALL_MAN which can easily have the target fixed to
use ${MANDIR}.

Some packages already set MANDIR which can be fixed to use ?=.

Some packages use INSTALLATION_DIRS which can be fixed.

Any comments?

I will attempt on a test system and let you know what I learn.

   Jeremy C. Reed
   http://bsd.reedmedia.net/