Did some more research.
Per the Joint Domestic Violence, Sexual Assault and Child Abuse Policy (the "Policy" - you can find it as Attachment 52 to the CBA), the first step (taken by MLB today) is to place the player on administrative leave. Under the Policy, when the player is placed on administrative leave, he continues to receive salary and is credited with service time - but he is placed on the Restricted List, which means he doesn't count against either the 25-man Active roster limit, nor the 40-man roster limit. Further, under MLB Rule 11, the club can activate an optioned player without regard to the 10-day minimum limit on optional assignments.
So yeah, they can call up Williams this afternoon if they want to. They could alternatively select somebody not currently on the 40-man roster, if they wanted to go that direction. I have to say, though, that if they ignore Nick Williams and select a Lane Adams, they're going to have one very disaffected ballplayer (and likely some rather annoyed players in the ML clubhouse, as well).