According to St. Thomas, one should always render judgment according to the written law, insofar as the written law does not violate the natural law (ST, II-II, q. 60, a. 5).
Earlier, in discussing the question of whether or not one must always act according to the letter of the law, St. Thomas argues that one can sometimes ignore what appears to be the letter of the law in favor of the intention of the lawgiver. Now, one objection states that only he who made the law can interpret it, the obvious reason being that he knows his own mind on the matter and what he meant when he wrote the law (ST, I-II, q. 96, a. 6 ob. 2).
In responding to this objection, St. Thomas argues that one who acts according to the intention of the law does not interpret it simply. That is to say, they are neither giving the law its meaning nor interpreting it for themselves. Rather, they are acting according to its meaning and interpreting it in light of the meaning intended by the legislator (ST, I-II, q. 96, a. 6 ad 2).
Now, the discussion of the letter of the law was focused on those who live under the law and do not have the authority to make or judge laws for the community. Yet this discussion points to two important points. First, interpreting the law is not just a matter of the letter of the law, but also of the intentions behind the letter. Second, when necessity does not demand immediate action in the face of certain risk, the interpretation of the law and judgment on whether or not a dispensation is in order is not something that can be done by any citizen, but only by those to whom the authority is given (ST, I-II, q. 96, a. 6c).
How, then, are those in authority to interpret the law? First, they must judge by the written law. Second, the written law is not simply the text, but also the intention behind the text. Thus one who has the authority to interpret the law and render judgment according to the law must seek to understand the intentions of those who wrote and promulgated it. Now, I am no great legal scholar, but this seems to be the very definition of originalism. Thus it would seem that St. Thomas position on interpreting and judging according to the law is the originalist position.