okay, this time around, there will be tweakable parts...

infrastructure/mk/bsd.port.mk

@@ -1,6 +1,6 @@
 #-*- mode: Makefile; tab-width: 4; -*-
 # ex:ts=4 sw=4 filetype=make:
-#	$OpenBSD: bsd.port.mk,v 1.1395 2018/04/28 14:22:05 espie Exp $
+#	$OpenBSD: bsd.port.mk,v 1.1396 2018/04/30 09:29:12 espie Exp $
 #
 #	bsd.port.mk - 940820 Jordan K. Hubbard.
 #	This file is in the public domain.
@@ -73,6 +73,7 @@ WRKOBJDIR ?= ${PORTSDIR}/pobj
 WRKOBJDIR_MFS ?= /tmp/pobj
 FAKEOBJDIR ?=
 NEW_UPDATE_PLIST ?= No
+UPDATE_PLIST_ARGS ?=
 
 BULK_TARGETS ?=
 BULK_DO ?=
@@ -2374,7 +2375,7 @@ _extra_info += DEPPATHS${_s}="$$(${SETENV} FLAVOR=${FLAVOR:Q} SUBPACKAGE=${_s} P
 _update_plist = ${_cache_fragment}; \
 	PORTSDIR=${PORTSDIR} \
 	${_UPDATE_PLIST_SETUP} ${_PERLSCRIPT}/update-plist \
-	-P ${PKGDIR} \
+	-P ${PKGDIR} ${UPDATE_PLIST_ARGS} \
 	-X ${_FAKE_COOKIE} -X ${_INSTALL_PRE_COOKIE} -X ${WRKINST}/.saved_libs --
 .for i in ${BUILD_PACKAGES}
 _update_plist += `SUBPACKAGE=$i make run-depends-args lib-depends-args` ${PKG_ARGS$i} ${FULLPKGNAME$i}