Peter, are you wanting the meforshim [commentary on Messianic halachic principles] to be limited to the New Covenant or may they range the Scriptures?
They do not have to be limited to the New Covenant but may be found virtually anywhere (e.g. Written Torah, New Testament, Talmud, commentaries, codes, etc).
Principles are value statement which may be found virtually anywhere. Halacha is not limited in the same way that Written Torah is limited. The source for Written Torah is strictly the Tanak and the New Testament. The sources for Oral Torah, while they originated from one source (i.e. Moses), have evolved over time and been disseminated everywhere. Thus, vestiges of the original framework of principles (i.e. halachic values) can now be found in lots of historical sources (e.g. OT/NT, Talmud, codifications of law such Mishneh Torah, Shulchan Aruch) and even some modern sources. Once we find these general principles then we can assemble them so that halachic authorities in Messianic communities can apply them to specific situations.
We can analogize the Oral Torah to something perhaps more familiar. The Oral Torah is similar to the Common Law. The Common Law is a set of historic case law. Here's how the process works. Whenever there is a new case, the judge will make a probabilistic inference based on inductive reasoning, deriving a general principle from a multitude of particularized prior decisions, and applying that general principle to the case at hand. In short, to find out what the precise halacha is for a particular situation, one must first locate the precedential decisions on the topic and analogize those decisions to the case at hand. Specific decision to general principle which in turn is applied to a specific situation.
The "prior decisions" can come in various forms. You might see something from the New Testament that, while not being a traditional halachic case, nevertheless reveals a general principle of halacha. Other sources for precedent include the Written Torah, Talmud and subsequent codifications of halacha.
Once the set of prior decisions is assembled, we induce the general principle. We then have to test the general principle to make sure is Constitutional. The analysis should look something like this: (1) does the proposed principle (set of values) bear an obvious relation to a legitimate goal of Torah?; (2) does the Torah (OT/NT) prohibit this principle (set of values) in any way?
Finally, we must discuss the difference between principles and rules:
"There are at least three seeming differences between rules and principles. The first is that rules lead directly to their conclusion if they are applied, while principles lead to their conclusion in two steps: first principles give rise to reasons, then these reasons are weighed before a conclusion is drawn.
The second difference between rules and principles appears in the case of a conflict. When rules conflict, i.e., when rules with incompatible conclusions apply to a single case, the rules lead directly to their conclusions, and therefore to a contradiction. When principles conflict, i.e., when principles with incompatible conclusions apply to a single case, no such problems occur. The application of conflicting principles only leads to reasons that plead for incompatible conclusions, so no contradiction is involved. In such cases, a conflict can involve several distinct reasons, some of which plead for a conclusion, others against it. Weighing the pros and cons determines the final conclusion.
The third difference is that rules are independent of other rules and principles and lead to their conclusion in isolation, while principles interact with other principles. For instance, additional reasons arising from other principles can influence the result of the weighing of the reasons," (An Integrated View on Rules and Principles by Verheij, Hage, and Van Den Herik).
In short, reasoning with rules and principles works as follows:
Rule (i.e. an "if, then" statement): if the condition applies, the rule applies.
Principle (i.e. value statement): rather than being framed as a rigid "if, then" proposition, a principle is a set of reasons which, when invoked, require further evaluation as to the relative merits of each particular reason from the set.