Thanks for input Rich,
I've achieved my goals with the following.
Added to /opt/bootlocal.sh
sudo syslogd -O /mnt/sda1/messages
sudo klogd
In case anyone else is interested, syslogd can be stopped with
sudo killall syslogd
I'm wondering if klogd and syslogd save output to the same file in microcore/tinycore ?