Thank you Richard
That is very helpful, though the article is slightly confusing as it says this issue was "fixed" in 7.2.1. However the link at the bottom
explains that starting in ONTAP 7.3.0, the WEEKLY_LOG messages (in general any ASUP message below ERROR level) are no longer sent to the email addresses listed in options autosupport.to (eg me). This accounts for my observation (and also explains why I thought my FAS2020 which has always run ONTAP 7.3 wasn't sending ASUPs - I assumed it was because it has standard rather than premium maintenance!)
The article also suggests a workaround using options autosupport.partner.to to specify email addresses that should receive all ASUPs. Messages to this mailing list are not filtered and so recipients will receive the WEEKLY_LOG messages.
Article http://now.netapp.com/NOW/cgi-bin/bol?Type=Detail&Display=152338 also mentions that you can configure My AutoSupport to alert you whenever NetApp receives an ASUP from your filers. That sounds worth exploring as my reason for wanting to receive the WEEKLY_LOG messages was to be sure our filers were sending them.
The conclusion is that you _can_ configure which ASUP messages you receive (though the procedure is rather arcane)
After about two hours of searching, I stumbled by accident on the place where you can set up customized notifications from My AutoSupport when particular types of ASUP messages have been received. You go to the main My AutoSupport page for a particular filer (the page that includes the storage efficiency 3D column chart and the performance graph). Then, right at the bottom, you will find a collapsed section with a link on the right hand side to Customize Notifications. I must have looked at this page dozens of times without seeing that. Why isn't it at the top, next to the Preferences and Online Support links? I looked at these two or three times hoping they might include something about notifications. Anyway, I've looked through all the available settings and I can't see any mention of WEEKLY_LOG though there is a setting for DAILY_LOG. I guess I could try setting that and then wait until next Sunday... zzz...