Thanks again to Christopher and Antonio for the help...
The good news is that chkuser is working as expected now.
The bad news is that I didn't change anything to make it so.
Presumably, my initial test must have been flawed, because
it appears chkuser has been running fine for the past 10+
Also, I found the logs, as Antonio pointed out -- qmail-smtpd
was logging to /var/log/qmail/smtpd/current (as specified
by /var/qmail/supervise/qmail-smtpd/log/run ), and that
file indeed contained full chkuser output. Unfortunately
the default multilog config (99k, 10 files) saves only
about 30 minutes' worth of qmail-smtpd logs, so I can't
figure out when chkuser came to life, but it must have
been last night because I haven't restarted qmail today.
Sorry for taking everyone's time on a non-problem... I
don't believe I changed anything since the failed test, but
I can't complain about the ultimate result.