I commented above about players joining the Brumbies' rolling maul in front of Poey, but exactly the same thing happened with the Highlanders' maul from which they scored as well. As noted by Cyclopath, most teams do it and it probably is not according to the laws.
Maybe the answer is that once the maul is formed (ie the ball is held by the player at the back), no-one else is permitted to join.
Some points about the Brumbies' game that I applaud.
The rolling maul was only used inside 10m from the tryline, ie it was it was used as a try-scoring move, not just to win a penalty.
The majority of kicks from the Brumbies' red zone were long and sent the Clan back into their own half. Some even found touch well into the opposition half, thanks largely to Moggy's huge boot. Quick throw in by the opposition is immaterial. They will overwhelmingly win the lineout anyway wherever it is contested.
More willingness to run the ball back after an opposition kick. Wingers supplied good support to Moggy though he seemed to want to run the ball himself rather than use them. But bloody great to see the amount of kicking cut right down.
Very few box kicks. Can only remember Dowsett putting one up, which went into touch on the full so it wasn't a good one.
Scrums were very meh. Brumbies seemed to be free kicked or penalised in most; including a couple of early shoves and three or four hinging or collapsing. One scrum the ball was held at the back for a penalty to the Brumbies, and another was held until the ref called them to use it. Thought JP should have been replaced by late in the first half as it was obvious he was the (only) target for the ref in ruling on the scrums. Ben A could have come on at LH if it was thought Alan A wasn't yet up to such a long period of play at this level. Could then have swapped when Ruan left later in the game (assuming he wasn't replaced early due to injury).
Best for the Brumbies were Poey, Fardy and Vaea. TK was good for his first game back from injury. Christian and Dowsett were both better than last week. Henry looks better at wing. Moggy was the real surprise given his lack of game time this year. Put in a great effort and didn't shirk any of the tough work. Keep it up.