i'm not sure where your statement about using hipaa to cover up substandard care comes from. it sounds like you assume that's the reason that family members are asked to leave. just one of many problems that come up in regards to this topic (family assuming this).
as far as whether your husband is under the influence of medication or not...doesn't matter. sorry. if i ask him if he wants you in there, and he says "no," the answer is no, no matter what meds he has received. i'm not going to hear him say no, and think, "well, if he wouldn't have gotten morphine he would've said it was okay, so i am going to let her stay anyway." not gonna happen. for whatever reason, at that moment, he doesn't want you in there, so that's the way it's going to be (i'm not saying your husband has ever said that, i'm using it as an example since you did).
honestly, no....having you designated as his poa means nothing. the poa isn't activated, for one thing, so you still have no rights to his info. the act of simply designating you as his poa for the day in the future that he will need one isn't enough to speak to the "trust" he has in you or his "intentions" (thereby including you in every discussion). i am designated as my mom's poa, but that doesn't mean she wants me in the room all the time, especially if the conversation with the nurse or doctor is of a sensitive nature. that's her choice to make at the time the discussion occurs. plenty of married couples designate each other as their poa's. later on, they get divorced. having a copy of the patient's poa on the chart doesn't mean i'm going to see it, think, "the patient designated this person to make decisions for them in the event that they can't, so that means i won't/don't need to ask them to step out for anything." that's a leap, sorry.
you asked: "what would i need to do, having supported my husband through major illnesses, to convince you that i know he would want me to be present?" honestly? if at that moment, i ask the patient if he wants you there and he says no....the answer is nothing. there is nothing that you can do or say that will override what the patient says they want at that time. if he is unable to answer due to his physical condition, then yes, i likely will let you stay. if he is able to answer, even if he's had morphine, his answer trumps everything else. in cases of patients with advanced dementia, it's a little trickier, and those situations are evaluated on a case-by-case basis; there are too many variables. receiving pain medication is not the same as having dementia, though. just because they've been medicated doesn't mean they don't know what they want. if later on, he says to you, "i'm sorry i said no, i didn't realize" or some such thing, well....i'm sorry. that's between you two. i have to err on the side of the patient, though. it's the right thing to do. much preferable to him later saying to me (or someone else), "i told the nurse i didn't want my wife in the room while they did x, y, and z, and she let her stay anyway." not a risk i can take. it's not personal. i don't think less of him or you, if he would want privacy for some things. i'm not "judging" your relationship based on that.
you could try to get an attorney involved, but it would be a waste of your time and money. there is nothing wrong or unlawful about honoring a patient's wishes, even if those wishes change from one day to the next. i think you'd be hard-pressed to find a judge who would say otherwise.
your last statement: "i think that as nurses we should be facilitating that support system whenever possible, while respecting the patient's wishes." exactly. if that patient is your husband and his wish is that you leave the room, i'm going to respect that. i'm not saying that he would ever say that, mind you. i don't know him or you. the same would go for any patient.