But in truth, I tend to doubt that it was within the established social norms of the class and period. I have seen period photos of women with their hair down, but photos of First-Class ladies in 1912? Well, most of the photos of them tend to have been taken when they were 'dressed up' in their best; so I don't imagine they wanted to abandon their hair fashion. Hats were worn so much, and they were never to be taken off outdoors I believe.
I'd love to hear some answers though; I'm still trying to research proper 1912 etiquette.
I'm researching this trying to find period documentation, but all I can find so far are little references here and there. It was certainly the style of fashion at the time, and I believe having the hair down while wearing any dinner dress was as much a fashion faux pas as it was impractical. The etiquette books I searched in do call for the hair to be dressed up in nearly all the situations it is brought up. I'll keep looking.
I just hope the same thing didn't apply in steerage, and that Third Class girls were allowed to have their hair down. Rachel's seductiveness in my Titanic book on Wattpad is largely thanks to her striking long, violet hair.