In the original video, I think that there was definitely no ruck formed, so it was a tackle situation. Therefore, the Province 12 did not have to come through the gate. In essence, he just joined the tackle.
The province 12, though, broke two laws (too lazy to look up the numbers):
1. Tackling a player on the ground
2. Preventing the tackled player playing the ball
The issue now gets clouded because the Lions player was in the act of scoring a try. The first rule he broke - tackling a player on the ground - is frequently overlooked when a player is scoring a try, generally as there's no other way to stop a sliding player. So let's let that one pass for now.
The second is a grey area. I am uncertain as to how they should handle it, as I think that players SHOULD have a right to get involved and prevent a try in certain situations, but under the laws, they do not. My own feeling is that it was not a penalty try.
In the O'Driscoll / Read case, O'Driscoll was on his feet as he played for the ball and Read then places the ball on the other tackler's chest. BOD then continues to play the ball off of his feet. This one is unclear to me.
I sorta agree with
Dam0 that I wouldn't want too many of those ending up as a penalty try.