OK, complete and total replication. Ran the routine on two more ION 3000 devices, verified the disablement of bypasses by bridging all eight ports on the expansion board into a single network, connecting multiple devices to those ports, and accessing one device from another, including making the remotely accessed device access the Internet.
Found a small refinement: you can actually install pfSense on the device (no need to resort to installing on another device), as long as you remember to use the right settings.
Anyway, here's the refined procedure:
One. Turn off the device (completely, as in, disconnect the power cable) and open up the case (there are four screws holding the top in place, one on each side and two in the back; once you remove the screws, the top slides off).
Two. Disconnect the SATA SSD and remove the CF card.
Three. Reposition a jumper on the expansion board and do some OpenWrt magic. For details on this step, see my post on the OpenWrt forum:
https://forum.openwrt.org/t/report-openwrt-on-cloudgenix-ion-3000/246014
Briefly, you prepare a bootable device with OpenWrt (I used a CF card, but there's no reason you can't use a USB stick), boot the device from it, install three additional packages, make a slight edit to /boot/grub/grub.cfg to configure the disablement of bypasses, reboot the device (which should disable the bypasses), let the boot sequence run, then shut the device down.
Four. Remove OpenWrt media, reconnect the SATA SSD, connect a USB stick with pfSense installer, and install pfSense. Remember the settings: UFS file system, entire drive, MS-DOS partitions (the layout suggested by the installer should work).
Once the install is complete, shut the device down, disconnect the installer media, and boot the device. It should boot from the SSD, and bypasses should be disabled, so you can assign interfaces and as usual.
Remember, the port map is weird:
= Expansion board =
====== Onboard controllers ====== igb6 igb7 igb8 igb9
igb10 igb11 igb12 igb13 igb0 igb1 igb2 igb3 igb4 igb5
so by default, the rightmost onboard port (igb1) will be LAN and the one to the left of it (igb0), WAN.
Five. To get the little LCD screen working, install lcdproc and configure it to use parallel port (/dev/lpt0) with 2x20 screen size and the Watchguard Firebox with SDEC driver.