I just had a dream last night, in which in one of the event's upcoming chapters, Sita appeared as one of the trials summoned by Indra, specifically to challenge Rama, and would stick around until the end of the event. She even had a special NPC sprite in battle, like Naucrate or Galahad, implying that she might not become playable right now, but she will be in the near future. Maybe they might release a bunch of new gold Servants for the anniversary, maybe she gets released much later, who knows?
All I know is that it was too good to be true.