How far does “free kick” go in taking a PK in time extended solely to take the PK? In regulation time a team can legally have a choreographed play whereby the kicker taking the PK passes the ball forward to an oncoming, not encroaching teammate – who finishes with a shot on goal. Could this be done in time extended solely to take the PK, or is only a direct one-time shot on goal allowed?

USSF answer (September 8, 2011):
The PROCEDURES TO DETERMINE THE WINNER OF A MATCH OR HOME-AND-AWAY, listed at the end of the Laws of the Game, tell us, “Unless otherwise stated, the relevant Laws of the Game and International F.A. Board Decisions apply when kicks from the penalty mark are being taken.”

Penalty kicks, once awarded, are taken regardless of the amount of time remaining in the half. If time expires or will expire before the restart can occur, the referee should announce this fact and indicate clearly that the penalty kick is now being taken “in extended time.” This means that no player other than the kicker and the opposing goalkeeper may enter the penalty area before or after the kick is taken by the original kicker. Therefore, no trick play such as you theorize would be possible.

Even if the second player did break the Law and enter the penalty area illegally, this excerpt from Advice to Referees 14.8 would also apply:

At the taking of a penalty kick in extended time, violations of Law 14 are handled the same as if the kick were not in extended time but with the following exception: if the required restart after a violation would be an indirect free kick, the kick in extended time and the period of play are considered over.