I have no idea when or if they will be announced ahead of time.
Ref's are selected based on ratings, recommendations by conference officiating folks, and in general are supposed to be among the best.
They rarely work the Elite 8 round, IIRC, so will be interested to see if Dee Kantner works tonight.
One comment - I was looking at who the refs were for the Baylor "fiasco" and found they included Melissa Barlow (a well respected ref that works the Big 12 and PAC, among other conferences, and has worked the FF4 in the past), Laura Morris (works a lot of shifts, including a bunch in the B1G I think) and Frank Steratore, who works the BE among other conferences. These ARE well thought of refs, regardless of how folks thought they performed. And there-in lies the rub - what do the recommending folks use for criteria.
If you don't mind detouring to the men's side, google Ed Rush / Sean Miller / PAC12 and see what I mean.