This is something I've been wondering a lot about since the new laws came out as well.
My personal opinion would be in agreement with CrossRef and Christy. I would think the offsides line can only be set by those participating in the ruck (as per usual) and now in this case with a mono-ruck it would be based off of the single rucker.
1) Initially I was thinking maybe the foremost foot of the single rucker would make the most sense (for consistent verbiage) and since every player only has 2 feet. But then you run into the possibility that the mono rucker is just leaning over the ball (both feet behind it still), if we agree that this constitutes a ruck per the new laws, then the opposing team's offsides line is passed the ball. This is not really a bad thing from our perspective, in fact it might encourage the mono rucker to actually step over the ball more so and set that offsides line for his opponents further back. In fact it might even encourage more players of the mono rucker's team to join in so they can drive him further forward and set the opposing teams offsides line back even further. The more I think about this, the more I like the tactical advantage of this at the cost of committing more players to the ruck.
Additionally you can run into the situation that the mono rucker stands with both feet in line with each other and therefore the offsides line for both teams is in line (overlap) each other. Again I don't think this is the end of the world and probably encourages better form from the mono rucker ultimately.
2) If not by the feet of the mono rucker, then perhaps the foremost part of the body the mono rucker. This might even make a little more logical sense because it instantly ensures there's a distinction between both teams offsides lines. Though I would feel it might be a little harder to judge exactly where that invisible line is for the team opposing the mono rucker (for both the official and the opposing players) but there's always the motto when in doubt play it safe with an extra half step back I suppose. Also I can see a mono rucker changing his opposing team's offsides line a lot easier without actually moving his feet or other players from his own team committing to the ruck and driving him through, by the sheer ability to freely change the angle they're leaning over the ruck. I would find this a bit weird and don't really care for it much but I also don't think it's something that would be common occurrence.
3) Finally the only other thing that would make sense to me if not based off the players in the ruck, then perhaps the ball would form the offsides line for the opposing team of the mono rucker. This would shift the paradigm a little bit because now you run into situations where the opposing team of the mono rucker can line up quite far passed the mono rucker himself, so long as they don't pass the ball. By this kind of law application, the new laws are still in favor of the opposing team of the mono rucker. I don't think that's a bad thing, because they still lost the ability to gather around the ruck without committing to it like the Italian's were doing and presumably what sparked this law change.
It also makes it a little interesting from the mono rucker and his team's perspective because if the opportunity presented itself and he read the field correctly, he may find the opportunity to quickly pick up the ball from between his feet and run straight through the ruck for a pretty clean breakaway if the opposing team is all lined up passed him already.
In conclusion I think any of the 3 ways to apply the law have both positives and negatives. I think in the spirit of the game, what World Rugby's intentions are, and what makes the most sense from both teams' and the official's perspectives is to apply the law by my #1 set of points. But I don't think there is any official ruling on this yet, so unfortunately we have to wait to hear back from the guys upstairs.
