You mean in your experience or based on experimental data? Hard to see how they could survive suffocation there. Timing is everything, as I realize you know but oil is considered an effective suppressant here and spring application has suppressed it for a whole season here at several sites I’ve managed over the years. .
Here’s what UC Davis says,
Monitor for San Jose scale during the dormant period by checking prunings to make sure scale hasn’t developed in tree tops. Also check fruit at harvest for the presence of scale.
Due to the damage potential of this pest, annual dormant sprays are recommended in most areas. Oil sprays work the best on the black cap stage, so apply them in early January. Control heavy populations of San Jose scale by applying an insecticide plus oil spray during the delayed dormant period.
If inadequate control is achieved with the dormant spray, treatments are also effective when applied soon after the emergence of the crawlers in May. Use pheromone traps in March to monitor for male San Jose scale flights and double-sided sticky tape for monitoring crawlers in April and May. Time a treatment, using a 51°F lower threshold and 90°F upper threshold, for 600 to 700 DD after the beginning of the male flight or 200 degree-days after crawler emergence begins.
I’ve never tried controlling scale on magnolias, so maybe that’s a different ball game than San Jose scale on apples.