I need to think some more about the voluntary aspect of moral structures... As regards isolation it's clear enough that there can be a voluntary element: one chooses to accept or decline an invitation to some gathering, for example. One resolves to become a hermit. One chooses to entrust others with some piece of sensitive information, or not. This last example reflects a kind of epistemic isolation; and I wonder whether there's some sense in which one can be even more subtly isolated, or not: that of whether at root one feels 'one of us', whatever anyone's actions might suggest about the matter.
That is something else I definitely need to deal with: the actually fairly flimsy connection between the fact of being alone and the understanding of one's situation in those terms. The loneliness of crowds, and so on. (There's also the fact that isolation as a long-term state can survive encounter: Philip Koch has quite a lot to say on this in Solitude: A Philosophical Encounter. Incidentally, yes, I'm using the term 'isolation' in a manner of which he'd disapprove.) Which I think is digestible in terms of a phenomenological us/other. If you're on a desert island and feeling the absence of any other human being, then any human presence will serve as the 'us' presence that breaks the sense of isolation. Whereas if you're a spy, keeping the truth of who he is from those around him, then those others can never be the 'us' presence that dispels the isolation (unless you decide to switch sides); indeed, it seems reasonably intuitive that the more you get involved with them (creating an 'us' that acts to oppose their 'other' status), the greater the sense of isolation will get.
Back to voluntariness... This doesn't obviously sit well with an us/other structure, which on the face of it looks perceptual and therefore non-voluntary. (As I said before, there are facts about what your connections are that are just facts: you can't rewrite what schools you went to, for example. Forging your C.V. doesn't really count.) Questions about the extent of the moral community, circle, etc. tend to be framed in terms that suggest the answers are to be (rationally) discovered rather than chosen; if we inject choice in there the first thing that comes to mind is personal relativism. This, however, is where I think I can employ Kierkegaard's sense of realising the universal.
The universal is discoverable inasmuch as one can discover what it would be to realise the universal, but precisely because it can be realised, or not, it leaves room for voluntariness. Disclosure is an ever-present possibility for the spy and even for the castaway, inofar as the castaway can choose to live such that he could in principle justify his actions to any audience with which he might come into contact: justification and argument being inherently public practices. (Maybe there's some room for referencing Stuart Hampshire's suggestion that our understanding of rational thought is derived from our grasp of public arbitration: 'The relations between the public activities of deliberating and adjudicating are open to everyone's observation, and their shadows, the corresponding private mental activities, are assumed to duplicate these relations.') So living non-gratuitously is a kind of public/intersubjective practice irrespective of whether there's an actual public.
That still needs to be fleshed out—and hooked up properly to the phenomenology of isolation. The principal difficulty is that there's nothing that realising the universal particularly looks like... Obviously in Either/Or you have the extended Seducer's Diary versus Wilhelm's praise of marriage, but seduction is publicly intelligible (hence presumably the pains Kierkegaard took to emphasise the subtleties of his seducer), and the mere public act of marrying is eo ipso just that. Unless one does have a case like that of Abraham to work with, pointing to what people actually do won't be very helpful; even if you do, do we have a clear idea of what the Attunement is supposed to show...? (Not the way I remember it from first year Reading Philosophy.)
Well, there's nothing solitude particularly looks like either... One thing probably worth flagging up is the manner in which isolation can be liberating: Sartre captures some of it, but basically in contradistinction to how the park looks once the Other has appeared. Out of the public gaze one is liberated to do... whatever one sees fit. Yet as I said above, there's a kind of seeing fit which is nonetheless public, i.e. one understands oneself as an agent in the public moral world. So there's another sense in which isolation also illuminates one's own personal responsibility—which again sounds quite Sartrean. (No refuge—from one's own eyes...) Except that Sartre emphasises choice (leaving aside the argument about whether he encouraged gratuity). So we've got this strange tension between the possibility for spontaneity and the correspondingly hightened possibility for adopting public standards.
This is where a case study would be really useful... Well, suppose I'm walking alone in a lush, quiet forest, and suddenly I come upon that common feature of the modern British woodland, the discarded plastic bag. A normatively construed aberation: that which intrudes where it has no place; moreover, someone has discarded it, i.e. its presence discloses conscious agency, responsibility, culpability. (Perhaps a betrayal or dereliction of duty with respect to our shared public space...) So now suppose that I have emptied my own plastic bag (having eaten my lunch), and the possibilities are open to me of discarding it or carrying it home. There is also the prior possibility of understanding this decision in terms in which I appear as the discarder of the other plastic bag appeared to me: and once I have recognised that possibility, even though no footsteps are approaching and so no actual Other is coming to fix me in his disapproving gaze, nevertheless I recognise my act as a public one—and now the possibilities open to me are of acting in the manner of a responsible person and taking my bag home, or discarding it anyway; I can no longer 'just' choose whichever suits me.
Something like this can be the case even for the 'last man', even though nobody else will ever see the plastic bags he discards, and he may enjoy the colour they lend to the landscape. He may judge that it no longer matters whether he litters, but this judgment is made already within the ethical, i.e. it involves his addressing the question of what responsibilities he bears or does not bear given certain possible public/objective standards. If, that is, he's realised the universal.