Why can't a few new targets be added to mk.bsd.pkg called
something like "make tag_for_update" which will simply
store the current package's directory (eg, print/tex) into
a data base.  You would run this from any package which you
want updated in the future.  You wouldn't want this flag
recursively carried into the build of dependent packages,
since dependancies for a package may change in the future.

Then, when you download a new pkgsrc.tar.gz from current,
you would go to its root dir, and type
"make update_all_tagged_for_update" which would simply enter
each directory stored in the data base made above, and do a
"make update" or whatever is appropriate.

The approach of storing the package's directory is version
independent in most cases, as far as I can tell.