The RFL rules section A1: 11 (j) states:
"Unless otherwise determined by the RFL, in each match in the League One play-off series, the highest ranked club shall have home advantage."
Basically, the only way we can be at home in the final is if Hunslet or Workington get to the final.
Should we lose to Newcastle, we'd then play the winner of the Hunslet v Workington game at home. If we were to knock them out, we'd then be the lowest ranked team left in, so couldn't play at home again.
It does seem somewhat harsh that we could win at Newcastle in the first game and then have to play them away again at a later stage but this is the price you pay for not finishing higher up the league.