BJ's speed would work well in the back row. His complete lack of a passing game unless its an offload means he should stay at prop. Besides that, can you really see him being an 80 minute back rower? He has trained for 20-30 minute bursts, not constantly running back and forth while occasionally tackling and running the ball for the full 80.
I'm not Brown's biggest fan but I do think this is probably the safest choice for Barrett and the side. He would know the team structures inside out (he should, he hasn't missed a game since he came here) and would have trained mostly as a back rower which is arguably his primary position. The fact that he can fill in at dummy half and centre has just meant he has become the teams utility player. While I don't think he's as good as Sirro has proven to be, remember that he did play a Grand Final against Manly in the back row and has played a number of tests for NZ in the position.