Hi there - first post, so be nice!
I'm surprised you think that an attacking scrum already moving towards the posts would never collapse it. Many moons ago, I played academy rugby as a hooker (the identity of the academy shall certainly remain nameless!), and we had a specific call which we used in the following circumstances:
a) attacking scrum on or close to 5m line;
b) our put in;
c) game so far showed us that we had the ability to get forward momentum.
In this scenario, we would make the call before the ball was put in. Then, when the ball was at 8 and we had started to make forward progress, the scrum-half would make a loud, clear call (in the form of a loud, pre-agreed comment to the referee). At that point, it was the responsibility of the front row to take the scrum down.
This worked on the following assumptions:
a) the referee was highly unlikely to penalise us in these circumstances. Worst case scenario, he would reset the scrum and we could either try something more positive or utilise the same call;
b) the call from the scrum-half was mean to distract the referee at the moment of the collapse (it took his attention to midfield), thus ensuring that he was very unlikely to see who had initiated the collapse; and
c) there was possibly a 10-20% chance of gaining a penalty and a shot at goal.
This was seen as a "shot to nothing" in snooker parlance, with effectively no down-side for us, and a decent chance of bagging three points.
In one and half seasons (before I was quite rightly released!), we were never penalised, and probably gained 10/12 penalties.
Negative, but effective.