I'd been looking, but hadn't seen much about some sort of historical precedence regarding Trey Zeigler's potential availability right away. But Jeff Eisenberg at Yahoo (with a hat tip to the Oakland Zoo, I might add), has an article that gives an example of a similar case - and the good news is that the player was eligible immediately.
Infielder Cade Kreuter transferred from USC to Miami after the Trojans fired his father Chad following the 2010 baseball season. A Miami spokesman confirmed the school applied for a waiver for Cade on those grounds and it was granted, enabling him to contribute right away in 2011 rather than sitting out a full year.
This is, of course, huge. Pitt will, and should, point this out when making their case to the NCAA to allow Zeigler to play right away. There's no guarantee that the NCAA will rule in Pitt's favor despite the earlier incident (this is, the NCAA, after all). We also don't know what other circumstances may have played a part in the decision. Still, it's a good example of the same thing happening and could be the reason Zeigler gets on the court a year earlier.