pine_beetleWhat's stopping the heavy from hiding in Z and blocking the cap in like 2 seconds radium? That's kind of the whole idea of the 4 man sac... Unless the scout delays his push the soldiers and demo won't be in position to damage the heavy yet so more likely you lose a scout without even forcing them. They will not need to commit an uber to that... He can just walk on the point and get a free kill. At which point your team might as well not even push in.
You said it right there. The scout must press the point as the demo/soldiers arrive in position. If he's too early then he's getting shut down. The scout is being used as bait - he is not going to force the uber.
What you COULD do if you hear a call that "heavy is in Z!" is to just have the demo come in from left shutter and lay down suppression fire onto Z. The key here - as aporia said earlier - is that SOMETHING needs to be controlling stairs/lunchbox and holding off their spawners from coming through a wide-open flank (they'll all come in right at the end of the engagement).