Redirecting console output to serial port to capture error message

[Roger L. Beeman]

I have been trying to capture the error message that I receive when
trying to boot any of the Release 5.3 beta or release candidate iso
images on my DEC Personal Workstation 333i+. The system is an LX440
chipset based dual processor (2x PII 333MHz).

I have been trying to redirect the error to the serial port and worked
a while to get to where I learned that this incantation:

#mkisofs -U -R -V fbsd_bootonly \
-no-emul-boot -b boot/cdboot -c boot_catalog \
-o /home/mbcd.iso /home/bootcd/cdrom

after adding a /boot/loader.conf.local file containing
  console=comconsole

to the contents of the bootonly.iso copied to /home/bootcd/cdrom would
actually redirect this much of the output to the serial port:

/boot/kernel/kernel text=0x4363cc data=0x7cc00+0x4fe20 
  syms=[0x4+0x5a690+0x4+0x6f5d8]

but after that, the error message still pours out only on the video
console, repeated endlessly, rolling by much too fast to make any
sense of.

What am I missing about redirecting the console output? I'm reading
section 20.6.6.1 of the the "fine" manual, as saying that this should
do the trick.

Roger L. Beeman