After a bit of trial-and-error, I found that the second stage boot
loader for i386 for /usr/sbin/installboot is /usr/mdec/biosboot.

Either the man page for installboot needs to mention biosboot or
better, biosboot should be installed as /usr/mdec/boot.i386, as the
man page indicates.

Also, step 1. in the man page (Copy the secondary bootstrap...) should
tell the user to what filename /usr/mdec/boot.MACHINE or
/usr/mdec/boot should be copied to. On /i386, /boot works for me.

After the initial hurdle, I now have the new boot blocks installed and
configured with serial console on the second com port!

cp -p /usr/mdec/biosboot /boot
installboot  -o console=com1,speed=9600  /dev/rwd0a  /usr/mdec/bootxx_ffsv1

Great work!
-- 
    - aew