Can back iron on a DD outrunner be reduced to save weight without a negative effect on efficiency as long as there is still no magnetic field measurable on the outside of the ring?
If it doesn't affect the return path of the fields, it shouldnt' change the motor operation.
It's been done to various outrunners over the years, including hubmotors (IIRC some middrives using DD hubs, perhaps even the original stokemonkey, with their spoke flanges machined off along with as much backiron / shell as they could get away with).
Of the DD hubmotors I've worked with that have a separate steel ring for backiron with an aluminum shell and spoke flanges, the backiron ring is typically 5mm or less. (Stromer's UltraMotor, early 9C, etc).
If the DD outrunner is a hubmotor, and the backiron is also the rotor shell and the spoke flange: Removing metal will weaken this structure and make it more susceptible to deformation from spoke tension, and dependng on how the machining is done between and next to the flanges, could even weaken the flanges themselves and if spoke tension is sufficiently high, allow failure of the flange to remain attached to the rotor.
This has happened without such mods for other reasons to some DD hubs over the years, though I think those were aluminum like some of the old 9C "500w" hubs, which had a separate steel ring inside the alumimum outer shell and flanges; there are also all-steel versions of them. (it has also happened to regular bike hubs, also for other reasons).
Additionally, for motors where the backiron / shell is a certain thickness to prevent deformation of it from the pull of the fields on the magnets attached to it (probably not the case for DD hubs) then thinning that increases deformation potential, and could not only cause changes in the fields because magnets can then move closer to stator teeth, etc., but also allow magnets to scrape the stator, if the deformation is large enough and the air gap started out small enough. I don't imagine either of these is much of a problem, but it might be worth considering for thin-shelled motors (some RC outrunners have shells so thin they have significant flux leakage outside the shell, according to some posts over the years--I think they might be in Kepler (or other ) friction drive threads, regarding "garbage" picked up and stuck to the outside of the motor shell, causing grip problems where the shell is directly used to drive the tire).