I suppose it depends on who she is.
If she was playing, for instance, Chapel, then I would find that unlikely to be a plot specific spoiler probably and it could likely be revealed without giving away anything else.
IF, however, her identity is inextricably tied to who Cumberbatch is playing and revealing one almost certainly gives away the other, then probably they will sit on it until they are ready to finally confirm who he is playing first.
They might do it differently but I think if I wanted to protect the secret of who he is, I would not confirm prior to that another character that would give it away.
'If the Apocalypse starts, beep me!' - Buffy Summers
'The sky's the limit.....' Jean-Luc Picard, 'All Good Things'
courtesy of Saquist