Your constraint should be that if the board is the special case of regular chess, the game should play exactly the same. Allowing the pawn to move towards any side is too much because it would violate this principle.

Perhaps define forward as the direction(s) which, when extrapolated as a rook, would reach the opponent's backrank. If it doesn't exist, the direction(s) which, when extrapolated, terminate on the opponent's half. This would allow extra freedom to pawns in some funky topologies that can occur in your game, but generally follows the principle of least surprisal.