TM -
I too have concerns about when to call. I'm probably going to take the route of stay quiet regardless of what I THINK the gun condition is,
until shooter stops at next position. Then watch to see what pops out when the slide gets racked. If a live round comes out upon racking, well, SDQ called then.
If no round, remember a Minor Safety that will be given at end of string. No way for shooter to correct it during stage, once he takes steps. So no need to call anything out loud for the MSV until the end.
Don't just look down to see if a live round is on ground to tell difference between the SDQ and MSV call. It could well have popped off the top of a magazine being drawn from holster and loaded.
And, unfortunately, the spotters probably will rarely be able to help the TO to make the call. It will all hinge on the TO's ability to position himself to the right side of shooter as the shooter comes to a stop, and to be WATCHING the gun as slide is racked.
Now, we will also want to speak up if shooter lays down the 1911 with slide forward. Shooter has until next gun fired to correct that. A verbal warning would help shooter avoid a MSV for having re-staged an empty 1911 with slide closed.
I believe we will find this (TO seeing the slide rack clearly) to be a hard thing to do when RO'ing high speed shooters and when dodging around props. WR will make a good proving ground.

(Wish it had been put together with simplicity for shooter and TO in mind.)
If a TO does make a SDQ call out loud and realizes that was a bad call, do not be afraid to (ask for ability to give reshoot, which major matches are requiring, and then) give the reshoot for having interfered with the run.
Good luck, GJ