In RFU land, league games have separate regulations for rolling substitutions.
Regulation 13, Appendix 2.
The main ones to note in relation to this conversation are highlighted in the text below, basically a player interchanged can't take a kick at goal as his first action and it's at the referee's discretion to allow an interchange. This seems much clearer than the clarification:
[LAWS]24. Not more than two Player Interchanges per team may occur at any one time and may only occur during a stoppage in play and in all cases with the permission of the Referee.
25. A Player must enter the field of play on the halfway line.
26. A Player who is a replacement shall not be entitled to take a kick at goal until a passage of play has taken place since that Player took to the field of play.
27. A Player who is previously injured in the match may return to play as a Player Interchange provided the Player is fit to continue playing in the match save that a Player who suffers two injuries in a match which has necessitated that Player being replaced on each occasion is not permitted to act as a replacement following the second injury.
28. In addition to the powers set out in the Laws of the Game, Referees are entitled in their sole opinion to prohibit or postpone a Player Interchange if they believe either that the Player Interchange would prevent the opposition from restarting the game quickly or where the replacement Player is not fit to continue playing in the match.
29. Under dispensation provided to Unions by World Rugby, where these rolling substitutions Regulations conflict with the Laws of the Game these Regulations shall take precedence.[/LAWS]