My problem was indeed a missing /etc/boot.conf. Creating that with contents stty com0 115200 set tty com0 solved my immediate problem, and editing /etc/ttys to have a getty running on the serial port (at the right baud rate) got me a fully working system.
Thanks to everyone who responded (both here & by private email) for reminding me about boot.conf -- I knew about that but had brain-parity-error overlooked it. ciao, -- -- "Jonathan Thornburg [remove -color to reply]" <dr.j.thornb...@gmail-pink.com> on the west coast of Canada "Now back when I worked in banking, if someone went to Barclays, pretended to be me, borrowed UKP10,000 and legged it, that was `impersonation', and it was the bank's money that had been stolen, not my identity. How did things change?" -- Ross Anderson