That mysterious sdtool you cannot find is hosted here, and you can indeed use it to enable temporary write protection on your card:
sudo umount /dev/mmcblk0p* sudo sdtool /dev/mmcblk0 lock sudo sdtool /dev/mmcblk0 reset Similarly, unlock will remove the write protection.
A word of warning: my system is not able to recognize that the flag is set, so it's up to me to remount the filesystem as read-only. Mounting write-protected card as rw will silently allow writes which in reality won't take place, resulting in weird filesystem errors (which fortunately won't affect the SD card but still can hand up programs on your computer).