Nandnor:
I don’t think the question you are asking here is that difficult, but it’s not exactly the same as the question you asked in the OP, which mentioned justification.
The reason one might keep a libertarian ethical argument in one’s head is because they have to make a choice in a situation where the results of that choice are not immediately apparent, but can only be known through abstract or theoretical knowledge. In some circumstances, one doesn’t know which choice or path is in one’s self-interest without some theoretical framework to guide one’s choices. So we can consider “the libertarian ethical argument in one’s head” as the particular theory each libertarian is working with, that instructs him/her that following xyz path, or making X choice in a given circumstance, is in their self-interest.
For example, we seem to know whether driving in the right or left lane of a two lane highway is in our self-interest. There is nearly unanimous agreement on this type of choice.
But there is less agreement on whether attending a government university is in one’s self-interest or not. Apparently, abstract reasoning is needed to make these kinds of choices. One has to keep a particular theory “in one’s head” as an aid in making choices where the results of the choice are remote, perhaps counter-intuitive, not immediately apparent, etc… (Menger calls it “knowledge extending beyond immediate experience”)
The reason that libertarians, utilizing libertarian social theory, generally try to abstain from or avoid things such as: making money from the state, using state power for self interests, exploiting the non-libertarian aspects of intellectual and regular property, etc., is that libertarian social theory tells them that doing such things is counter to their own rightly understood self-interest. Some of the consequences of doing these things are not immediately apparent. Some of the consequences are remote—beyond immediate experience. Thus an abstract theory is required to understand the consequences of doing these things.
So understanding why libertarians refer to an intellectual construct or theory that guides their actions is not particularly difficult.
But this is not the same thing as the question as to what “justifies” libertarianism. When we say “justify,” we now introduce intersubjectivity…
Question: What is your justification for what you just did? Because I’m not happy about it, and am considering retaliation.
Or again: I believe your expressed plan will have “bad” consequences (consequences detrimental to my interests). Thus: Justify your plan to me. Tell my why I should support it.
That is “justification.” It deals with intersubjectivity, usually intersubjective conflict.
Concepts such as justification and argumentation, as these concepts are used in libertarian ethics, are used, as Hoppe writes, in an attempt to establish “that intersubjectively meaningful norms must exist.” (TOS, p.130)(underline and italics added)
The question you ask above can be answered without referring to the concept of justification and a theory of intersubjective norms. Your question above can be answered in terms of the theory of action, in terms of methodological individualism, and whether the individual’s means can bring about the ends he strives to attain.
The question you ask in the OP, as it specifically asks about justification, introduces the concept of intersubjectivity, since the theories that utilize the concept of justification generally refer to a system incorporating the idea of intersubjectivity.