The history of Spanish conversos, from the forced conversions of 1391 until after the expulsion, is complicated and contentious. Nevertheless, while medieval rabbinic authorities broadly did not defend conversion to Catholicism, there are many teshuvot that deal with the practical implications of conversos (e.g., the status of children, marrying someone who was previously baptized, etc.). If you're asking about rationales for conversion from the very Jews who tried to retain their faith through conversion (as opposed to the majority of those who converted out of convenience/assimilation/secularism), there are various explanations.
Here is one rationale, from Rabbi Marc Angel's work on Sephardic history, Voices in Exile (p. 46-47). As Angel explains, crypto-Jews did not have many Jewish texts or authorities to rely on during the 15th century, so it's difficult to talk about them receiving "halakhic justification" for their crypto-Judaism. They did, however, have the Christian Bible (which not only contained the Torah, as the "Old Testament," but also books of Apocrypha), which they used for theological support:
The theory of crypto-Judaism found support in the Epistle of Jeremy, the last chapter of the [Apocryphal] Book of Barukh: "When you see a multitude before you and behind bowing down, you shall say in your hearts: You alone are to be praised, O Lord" (6:5-6). Conversos applied this lesson to themselves, holding that true worship of the Lord depends on one's heart, not on external actions. Converso theology also found precedent in the biblical Esther. She kept her Jewish identity secret from her husband, King Ahasuerus, conducting herself outwardly as a Persian queen, but inwardly remaining faithful to Judaism. The story had special appeal to the conversos because, after all, Esther was a heroine. She ultimately saved the Jewish people from destruction. Her crypto-Judaism was, thus, vindicated.
Angel goes on to detail the spectrum of opinions by medieval rabbis on how to understand the crisis of conversos, as well as the efforts by Sephardic communities outside of Spain and Portugal who maintained contacts with conversos, and actively worked to help them return as open Jews.
The OP specifically asks about rabbis defending nominal conversions and crypto-Judaism. Considering the cardinal value against idolatry in Judaism, and the opportunities to leave Spain, contemporaneous responsa are overwhelmingly negative to the a priori idea of conversion (and while there are responsa that sympathize with the plight of conversos, I have yet to see any rabbinic authority defend 14th or 15th century conversion to Catholicism). Here is how historian Norman Roth summarizes the issue, in his work Conversos, Inquisition, and the expulsion of the Jews from Spain (p. 32):
The Jewish authorities of Spain already considered the conversos to be not anusim but meshumadim, not "forced" converts acting under duress but complete and willful converts who were no longer part of the Jewish people, and this in the fourteenth century, not just in the fifteenth.
One might suppose that the attacks on Jewish communities in the summer of 1391 resulted in massive conversions due to fear, like those of the First Crusade in Germany. Indeed, there is considerable evidence of mass conversion, but given the forceful reaction of the kings there was clearly no reason for "fear," nor did the rabbinic authorities treat those conversion as happening under "duress" but, again, as complete willful conversion. Furthermore, those who had the opportunity to leave the country and return to Judaism did not do so.
