I tried commenting that line out and compiled.
I have a 2gb SD card formatted with fat16. When I put in the locked card with the diskboot file it just starts normally and displays card locked.
I was told that the current version crashes. If you get a card locked instead with only this change, that means it's a step in the right direction. If you simply get "card locked" with any diskboot, it may not be loading at all (BOOTDISK string not in place, not encoded with dancing bits, or other problem)
Since starting normally using the PS.FI2 does not get around the SD write protect I suspect there are other things in the way including the write protect check and bypass
The write protect override is done in the keyboard task. See SD_READONLY_FLAG
You should be able to find this by comparing the values of physw_status with a locked and unlocked card.