Boy, good question. This would be a good one to ask over in the appropriate Postgres mailing list. Check out www.postgresql.org.
You could, of course, custom-build a copy that would put it wherever you want, but that's butt-ugly. There doesn't seem to be any command-line option to do this, and I can see why you don't want to symlink /tmp proper! You might suggest to the PG development team that /tmp/postgres or something similar be used rather than just /tmp.
