Thanks. Further digging shows that my current setup was as described in http://www.ijs.si/software/amavisd/README.postfix.old (which wasn't old when I first started using it, heh). I see that it has been supplanted by (the now 2-3 year old) http://www.ijs.si/software/amavisd/README.postfix.html which doesn't use the split cleanup and does use no_address_mappings.
Since even my copy of "The Book of Postfix" is going on 5 years old now, I find myself wondering what the current state-of-the-art is for setting up postfix, amavisd-new, et al ... -ste