While it's not the BSD you want, OpenBSD's bootloader can do this with an /etc/boot.conf file. All you need is a two line /etc/boot.conf file on the booting filesystem (optical, disk, or PXE). One line to set the console device to the first serial port, and a second line to set the speed to match the hardware:
Code:
set tty com0
stty com0 115200
Should you consider this as a possible solution, see the boot.conf(5) man page.