I have not tested this by running or using (just installing). I did this 
because doing installs as non-root.

Index: sysutils/dbus/Makefile
===================================================================
RCS file: /cvsroot/pkgsrc/sysutils/dbus/Makefile,v
retrieving revision 1.11
diff -u -r1.11 Makefile
--- sysutils/dbus/Makefile	2 Apr 2006 18:20:53 -0000	1.11
+++ sysutils/dbus/Makefile	8 Apr 2006 20:51:21 -0000
@@ -4,8 +4,8 @@
 .include "Makefile.common"
 
 PKGREVISION=	1
-PKG_USERS=	messagebus:messagebus::System\ message\ bus:${VARBASE}/run/dbus:${NOLOGIN}
-PKG_GROUPS=	messagebus
+PKG_USERS=	${DBUS_USER}:${DBUS_GROUP}::System\ message\ bus:${VARBASE}/run/dbus:${NOLOGIN}
+PKG_GROUPS=	${DBUS_GROUP}
 
 MAKE_DIRS=	${PKG_SYSCONFDIR}/event.d
 MAKE_DIRS+=	${PKG_SYSCONFDIR}/system.d
@@ -16,6 +16,9 @@
 CONF_FILES=	${EGDIR}/session.conf ${PKG_SYSCONFDIR}/session.conf
 CONF_FILES+=	${EGDIR}/system.conf ${PKG_SYSCONFDIR}/system.conf
 
+FILES_SUBST+=	DBUS_USER=${DBUS_USER}
+FILES_SUBST+=	DBUS_GROUP=${DBUS_GROUP}
+
 .include "../../devel/gettext-lib/buildlink3.mk"
 
 .include "../../mk/bsd.pkg.mk"
Index: sysutils/dbus/Makefile.common
===================================================================
RCS file: /cvsroot/pkgsrc/sysutils/dbus/Makefile.common,v
retrieving revision 1.14
diff -u -r1.14 Makefile.common
--- sysutils/dbus/Makefile.common	6 Apr 2006 06:22:45 -0000	1.14
+++ sysutils/dbus/Makefile.common	8 Apr 2006 20:51:21 -0000
@@ -43,6 +43,7 @@
 CONFIGURE_ARGS+=	--sysconfdir=${PKG_SYSCONFDIR:Q}
 CONFIGURE_ARGS+=	--with-xml=expat
 CONFIGURE_ARGS+=	--without-x
+CONFIGURE_ARGS+=	--with-dbus-user=${DBUS_USER}
 
 PKG_SYSCONFSUBDIR=	dbus-1
 
Index: sysutils/dbus/files/dbus.sh
===================================================================
RCS file: /cvsroot/pkgsrc/sysutils/dbus/files/dbus.sh,v
retrieving revision 1.3
diff -u -r1.3 dbus.sh
--- sysutils/dbus/files/dbus.sh	30 Mar 2006 20:57:08 -0000	1.3
+++ sysutils/dbus/files/dbus.sh	8 Apr 2006 20:51:21 -0000
@@ -14,8 +14,8 @@
 command_args="--system"
 pidfile="@VARBASE@/run/dbus/pid"
 start_precmd=dbus_prestart
-dbus_user=messagebus
-dbus_group=messagebus
+dbus_user=@DBUS_USER@
+dbus_group=@DBUS_GROUP@
 
 dbus_prestart() {
 	if @TEST@ ! -d "@VARBASE@/run/dbus"; then
Index: mk/defaults/mk.conf
===================================================================
RCS file: /cvsroot/pkgsrc/mk/defaults/mk.conf,v
retrieving revision 1.115
diff -u -r1.115 mk.conf
--- mk/defaults/mk.conf	6 Apr 2006 06:23:06 -0000	1.115
+++ mk/defaults/mk.conf	8 Apr 2006 20:51:22 -0000
@@ -691,6 +691,17 @@
 # Possible: any user name
 # Default: cyrus
 
+DBUS_GROUP?=	messagebus
+# Used in the dbus package to specify the daemon group.
+# Possible: any group name
+# Default: messagebus
+
+DBUS_USER?=	messagebus
+# Used in the dbus package to specify the daemon user.
+# Possible: any user name
+# Default: dbus
+
+
 DEFANG_GROUP?=	defang
 # Used in the mimedefang package to specify the group to run commands as
 # Possible: any group name