Here is the pertinent part of the Special Substitution rule;
Special Substitution
SECTION 23. Substitution may take place without the necessity of waiting for suspension of play by an official. One player at a time may enter the field of play from the special substitution area under the following conditions:...
d. The substitute must wait until such player is off the field of play, and the substitute may not delay his entry onto the field.
A.R. 109. During a special substitution, A1 delays his entrance onto the field. The trailing official sees the delay. RULING: Silent play on with an officials arm in the air unless A1 participates in the play, technical foul. This does not remove the responsibility of a team to adhere to the offsides rule.
This is designed to prevent players from delaying their entry onto the field to gain an advantage. With the frequent positional substitutions in the college game, this would stop a LSM from exiting the field and having the short-stick delay, and then pop onto the field to take a clearing pass. Conversely, a defensive sub delaying his entry and then delivering a body check would be a no-no.
In the first instance it would be a turnover, in the second, a 30 sec. TF if the other team has possession. If the player doesn't get involved in an imminent play, the play-on is waved off.
Just to add to the confusion, they have this A.R. all by its lonesome self under Unsportsmanlike Conduct;
A.R. 24. Team A hides its 10th man in the penalty-box area for the purpose of slipping him into the game during Team A’s clearing attempt. RULING: Personal foul, unsportsmanlike conduct.
I would think that most officials would go with the lesser penalty and reserve the USC for repeated violations, It does create another grey area that should be addressed.