Update: see this post for more on this.
I asked recently of some colleagues if anyone recalled where Mises said that economics or catallactics is the most developed or highly elaborated branch of praxeology, but that someone was working (at the time) on applying praxeology to the study of conflict, or war. Sudha Shenoy pointed me to the answer: in Mises' Ultimate Foundation of Economic Science Mises writes:
Up to now the only part of praxeology that has been developed into a scientific system is economics. A Polish philosopher, Tadeusz Kotarbinski, is trying to develop a new branch of praxeology, the praxeological theory of conflict and war as opposed to the theory of cooperation or economics.
 T. Kotarbinski, "Considérations sur la théorie générale de la lutte," Appendix to Z Zagadnien Ogólnej Teorii Walki (Warsaw, 1938), pp. 65-92; the same author, "Idée de la methodologie générale praxeologie," Travaux du IXe Congrés International de Philosophie (Paris, 1937), IV, 190-94. The theory of games has no reference whatever to the theory of action. Of course, playing a game is action, but so is smoking a cigarette or munching a sandwich. See below, pp. 87 ff.
Update: I stumbled across Adam Knott's working paper, Rothbardian-Randian Ethics and The Coming Methodenstreit in Libertarian Ethical Science. Knott writes,
Specifically, praxeology has not succeeded to date, in arriving at cause and effect laws in the social-ethical realm. In the strictly scientific sense as understood by praxeology, there are no known laws of ethical phenomena akin to the various economic laws established since the beginning of economic science several centuries ago.
Not sure why Knott does not mention (if only to criticize) Hoppe's work on extending praxeology to the field of ethics.
I'm not sure if Kotarbinski or anyone else ever completed this, or any other systematic study of a field of praxeology outside economics (and wouldn't Mises' and Rothbard's analysis of intervention in the market be a type of application of praxeology to conflict?). I did find this unusual paper by Alexander Mosely, Praxeology and Cultural Convergences in the Rules of War, but this does not seem to be apropos.
Interestingly, in his Reply to Schuller, Rothbard writes:
The categories of praxeology may be outlined as follows:
Praxeology--the general, formal theory of human action:
A. The Theory of the Isolated Individual (Crusoe Economics)
B. The Theory of Voluntary Interpersonal Exchange (Catallactics, or the Economics of the Market)
2. With Medium of Exchange
a. On the Unhampered Market
b. Effects of Violent Intervention with the Market
c. Effects of Violent Abolition of the Market (Socialism)
C. The Theory of War--Hostile Action
D. The Theory of Games (e.g., Von Neumann and Morgenstern)
Clearly, A and B--Economics--is the only fully elaborated part of praxeology. The others are largely unexplored areas.
See also MESPM, p. 74, where Rothbard writes: "What is the relationship between praxeology and economic analysis? Economics is a subdivision of praxeology—so far the only fully elaborated subdivision. With praxeology as the general, formal theory of human action, economics includes the analysis of the action of an isolated individual (Crusoe economics) and, especially elaborate, the analysis of interpersonal exchange (catallactics). The rest of praxeology is an unexplored area. Attempts have been made to formulate a logical theory of war and violent action, and violence in the form of government has been treated by political philosophy and by praxeology in tracing the effects of violent intervention in the free market. A theory of games has been elaborated, and interesting beginnings have been made in a logical analysis of voting."
Rothbard's mention of games apparently contradicts Mises's disparagement of games as a possible field of praxeology; and Rothbard's mention of the logic of voting seems a bit like public choice economics, but I am not sure. What other possible fields are there?
Arguably Hoppe's extension of praxeological type reasoning to the field of ethics might fit under Rothbard's category E, as Rothbard himself hinted at: regarding Hans-Hermann Hoppe's argumentation ethics defense of libertarian rights, about which Rothbard wrote:
In a dazzling breakthrough for political philosophy in general and for libertarianism in particular, he has managed to transcend the famous is/ought, fact/value dichotomy that has plagued philosophy since the days of the scholastics, and that had brought modern libertarianism into a tiresome deadlock. Not only that: Hans Hoppe has managed to establish the case for anarcho-capitalist-Lockean rights in an unprecedentedly hard-core manner, one that makes my own natural law/natural rights position seem almost wimpy in comparison.
A future research program for Hoppe and other libertarian philosophers would be (a) to see how far axiomatics can be extended into other spheres of ethics, or (b) to see if and how this axiomatic could be integrated into the standard natural law approach. These questions provide fascinating philosophical opportunities. Hoppe has lifted the American movement out of decades of sterile debate and deadlock, and provided us a route for future development of the libertarian discipline.
Interesting how Rothbard talks about possible extensions of praxeology as well as "axiomatics," the logical-deductive approach of Hoppe that is compatible with, if not a type of, praxeology.
Notice that two of Rothbard's books are Ethics of Liberty and The Logic of Action. Mayhap Hoppe's use of praxeology to investigate political ethics is a case of the Ethics of Action: a unification of Austrian economics and epistemology with libertarian justice. It is no wonder that, in drawing from the economic and methodological insights of Misesian-Austrian economics, Austro-libertarian theory is so powerful and sound.