Was that scene a meta-statement? Did Ennis show real depth of love and emotion for Jack because he was "in the closet" and hence safe from public scrutiny and view?
That's an interesting point of view. I had thought of that, in the sense that the two shirts were "in the closet", but I think Ennis wasn't hiding the two shirts, as Jack did, in a hidden corner of the closet, but in plain view, next to a postcard of Brokeback Mountain.
I don't recall if he opened the closet in that scene, but he put away Junior's sweater in it. Presumably, when Junior comes to visit again, he would fetch the sweater, and the shirts would be visible to Junior in that tiny trailer. I suspect Ennis wasn't hiding his love for Jack anymore, but simply keeping it private, without trumpeting it to the whole world, but also not denying it from his loved ones.
There is probably an unspoken understanding between him and Junior that he and Jack worked together one summer on Brokeback Mountain, and their relationship meant a lot to him. As others pointed out, "This Kurt, does he love you?" conveyed automatically to Junior that Ennis was also thinking of the love he lost.
So, in response to your question, I think that scene is indeed a meta-statement on the closet, and the state of Ennis being out of the closet to himself and his loved ones. The love and acceptance from Jack's Ma gave him strength, and comfort to be the man that he is, loving Jack. The fact that the door can still be closed to the outside world, is a statement that Ennis is not out, in the sense of joining the Pride parade anytime soon.
I don't know if that makes sense.