You are correct that the opened back gowns are immodest when one is placed in one and required to walk in public.
HOWEVER, as a person who works in the medical setting of a Hospital, It should be noted that Opened-backed hospital gowns serve a very specific purpose that of providing the medical staff treating a critically ill patient with immediate access for times of crisis (I.e. respiratory or cardiac failure), incontinence and cleanliness/inefection source, pain issues, and surgical needs. Can you imagine what it would be like to be in excruiciating pain in the ER after an accident, having to go to an Xray and having to remove clothing, jewelry, etc that would limit the value of teh Xray's evaluation?
The fact is that most hospitals I have worked in over the past 30 years provide an additional gown to slip over the back if one is expected to walk down a hall. When ever a person is able to wear their own robes, pajamas, they are allowed to do so.
Islam itself would usually find modern western medicine to be scandalous in that we allow male physicians to even be present at the time of treatment of women. Nothing we will do will ever satisfy their Islamic senses of propriety. Look at the example of modern medicine under the "highly correct" model of Islam that Afganistan Taliban offered its women---thousand died for lack of medicine---educated female physicians and health care professionals were eventually eliminated in all but maternity hospitals and even then prosecuted and prevented from doing the job due to lack of medicine or even the baisc of material to maintain sanitary conditions beyond water and soap.