Can you roll a locked story servant during the guaranteed gacha?

I would say, Yes.

Wait for the annoncement to make sure of that.

They were available in the JP anniversary, in fact the only servants that weren't in the the Guaranteed Gacha were the limited servants (Okita, Gil, Jalter, Etc.)

Edit: Nero was available during her event, even if you hadn't unlocked her through Septem, so I'm fairly sure they would be. It seems that they are only locked in the story gacha, and not in the unique banners that contain them.

If your servant is in the gacha pool, it doesn't matter whether the're storylocked or not