I've been putting a meaningless serial number on my subject lines since my original question, but it is hard to tell if it has made much of a difference.
One of my clients' server/isps has been inserting a [SPAM] tag into the subject line of my emails. It still happened with just the serial number, it still happened when I added a date, but it stopped when it put a [NEWS] header in front of the subject. Exact subject line follows.
The 80343 is just a made-up official-sounding number. It has no meaning other than to beat the system. I send out occasional press releases for different clients. There is no sequence, so a real volume- or Issue number is irrelevant.
I look at all the mail coming into this office, and I almost never see [NEWS] or [PR] tags. Do these really make any difference?