I have seen plaster rings screwed directly to the stud on interior, non-rated (the wall between the bathroom and bedroom in a hotel for example) walls for thermostats... We never use open back boxes or LV-1 boxes in commercial though, most walls in hotels are rated for fire and sound, and since 95% of our devices or in those walls, or ceilings, you must use a full or closed back box in those cases
You would need to look at the listing/instructions of the plaster ring to see if screwing it to the stud is permissible by 110 3B. Also have a look at 300.21. the rest of it would be in your local building codes.