Shiny locked Legendaries in ORAS
This streamer found a shiny Zekrom after about 600 soft resets
The video looks valid and on youtube you will find even more footage of other people who encountered a shiny version of these pokemon.
Someone in a comment section of a video I found pointed out that only story-specific legendarys are shiny locked but I'm not entirely sure whether this statement is true or not but if you're looking at the shiny locks so far, it appears to be true.