The Orioles recently designated Preston Guilmet for assignment. He's posted good minor league numbers, but he's 27 and has never had any success in the majors. I've seen him pitch for Columbus and have never been very impressed with him -- his fastball has averaged below 90 MPH in the big leagues. He's struck out better than a batter per inning in two years at Triple-A and has good control, though, so the Pirates may believe that gives them something to work with. They could also designate him for assignment themselves when they set their roster later this fall.
The Pirates shouldn't have much trouble getting Tabata back to the minors, since his situation isn't much different now than it was when they outrighted him earlier this year. There's still $8.75 million and two years remaining on his contract, and I'm sure the Bucs wouldn't mind if another team claimed him, which they likely won't.