I don't know if it would nail the various views which claim Jesus must reign on this earth, but it certainly does give them significant grief.
The phrase "if he were on earth" serves as a contrast to the writer's earlier statement, "We have such a High Priest, who is seated at the right hand of the throne of the Majesty in the heavens" (Hebrews 8:1). Jesus is High Priest in heaven, if he were on earth he would not even be a lowly priest since he is not of the tribe of Levi.
Scofield, a stanch dispensationalist and popular with the premillennialists, does even comment on this particular passage. He does mention in his comments on Hebrews 5:6 that Jesus was not made a High Priest by men but by God. Thus, I would conclude that Scofield avoids the literal meaning of this verse and only sees it as a statement of who appointed Jesus to the priesthood.