If you can't move the disk to a temporary machine with a console to do the installation, you could follow these guides to modify the ISO content, and generate a modified ISO, to be burned to a disk as usual or written to a USB flash drive:
http://forum.pfsense.org/index.php/topic,14763.0.html
https://secmachine.com/wp/how-tos/construct-usb-installer-freebsd/
You need a FreeBSD host machine to do this on, but I believe a liveCD would work. AFAIK, you need to add or change a line:
comconsole_speed="57600"
in the /boot/loader.conf file on the ISO. Not sure why it falls to 9600, since the loader man page indicates it should honor the existing speed:
comconsole_speed
Defines the speed of the serial console (i386 and amd64 only).
If the previous boot stage indicated that a serial console is
in use then this variable is initialized to the current speed
of the console serial port. Otherwise it is set to 9600 unless
this was overridden using the BOOT_COMCONSOLE_SPEED variable
when loader was compiled. Changes to the comconsole_speed
variable take effect immediately.
regards ….. Charlie