I got it to work.

I added:

/usr/sbin/tickadj 9200 2>&1 > /dev/null
/usr/sbin/rdate -s zaphod 2>&1 > /dev/null

to /etc/rc.local

That gets the clock speed and initial setting close enough that ntpd
or openntpd can do their job.

I determined the tick value experimentally. It's 10000 by default.
zaphod is my local time server, a real hardware machine running CentOS
and ntpd.