Okay, early report.
1) I survived.
2) It was a non-event. At 60MPH, 40MPH, and 20MPH I hit the kill switch, left it off until my speed had decreased by 5MPH, then switched it back on. Twisted the throttle and away I went. No bucking, no backfiring, nothing.
I don't know how it would be on a 400, but on the 650 the same forces that give us the strong engine-braking also seem to give us a smooth compression-start.
If I had slowed down enough for the CVT to disengage, at around 9MPH or so, it would have been another matter. Then I would have had to apply enough brake to disengage the starter lock-out, and then hit the starter switch to get the engine going again.
And once the CVT had disengaged, no matter how fast I was going, even accelerating down a hill, a compression-start wouldn't have happend because the engine wouldn't be turning, just the wheels.
So, my advice: don't hit the kill switch unless you really want to kill it. But if it should happen, don't panic.