You know, you could TRY flashing with S-ON. I don't know if the radio will flash, since I've never flashed a radio with S-ON. Try it.. what's the worst that can happen ? It'll error out ?
Or better yet, search around the forum and see if anybody else flashed a custom ICS rom WITHOUT s-off first. Hint.. look in mike's ROM and see if anybody was able to flash his firmware package without s-off.