Okay, so I did some investigation (mostly consisting of reading S20libromount script
) and here's the summary:
/opt, /opt1/keys and /opt1/info are mounted from cramfs images, meaning they're read-only.
/opt0 and /Data use jffs2 which is writable
/etc and /var are copied to ramdisk (created in /dev/shm/) and remounted at /, so we can't access the original dirs.
Sooo, the only way to change the files in /opt/ would be to change the actual cramfs images in the flash, which seems to be doable during the firmware update process. I will investigate how it's done but I don't think I'll be brave enough to attempt the actual reflashing