Using "the last out" as a courtesy runner is something that is usually only done when a contiuous batting order is being used. That is because all players are already "in the game". High school and ASA rules do not allow for a continual batting order. If one is being used, then you are not playing "by-the-book" rules- you are playing modified rules. And, it is the last batter to have batted but not reached base, not the "last out". The "last out" might have been, say, a runner trying to steal.
Here is the standard ASA Courtesy Runner (CR) rule:
- The CR must be a player who has not been in the game.
- May be used only for the pitcher or catcher.
- This does not count as a substitution (ie: the player being run for and the CR retain all re-entry rights).
- The same CR may not run for both the pitcher and the catcher at any point during the game.
- A player who was used as a CR may not be entered into the game as a substitute during the same half inning in which they were used as a CR (may come in later). Exception: If a player leaves due to injury or ejection, and no other subs are available, the CR may enter the game. If that CR is currently on base, the pitcher or catcher she was running for must take her place on the bases.
- The CR may not run for the Designated Player (DP) if the DP is batting for the pitcher or catcher.
- Once the CR is on base, the pitcher or catcher she is running for may not return to run while the CR is on base. Exception: Same as above for an injury or ejected player.
- A CR may not run for another CR.
- A CR must report to the plate umpire. If not, they are considered an illegal runner/unreported sub and disqualified from the game.
The high school rule is the same (unless I've missed some arcane twist), with the only exception being the unreported substitute penalty. In high school, a team warning is issued on the first offense. The player and the head coach are restricted to the bench on a second offense.
