Short answer - barring injuries, I can't see why Qld will mess with the formula regardless of how well Barba is playing. He will never usurup the Slater/Smith combination.
Wasted on the wings and would be a liability there in defence and under the high ball. Fullbacks make poor wingers.
If it's the first or second Origin game and Slater is out, I'd put the best 17 on the park and have Barba at 1 and Inglis at 3. If it's a decider, lock it down - Inglis at 1, Nielsen/Tate, strong defensive centres.
If he's named as utility on the bench, I think the only place you can play him is fullback, rest Slater and go for the throat around 25-30 minute mark before halftime in a 10-6, 12-6 Qld lead situation.
I'd only have him on the bench for Qld if Mal was prepared to do a Tim Sheens and not actually use him in a really tight, defensive game. And the forward battle is too intense to waste a bench spot on a trick shot.