The foundational rule stated by Shabbat 64b:16-18 and repeated in Beitzah 9a:6-7 and Avodah Zarah 12a:3-4 is that wherever the Sages prohibited something because of mar'it ayin, it is forbidden even in the innermost private chambers — the decree is not limited to cases where Torah law would otherwise be violated, which already suggests the concern operates broadly across rabbinic contexts as well.
Several codified applications of mar'it ayin in the Shulchan Arukh concern cases that are themselves only rabbinically prohibited, showing the concept is applied to דרבנן matters: Shulchan Arukh, Yoreh De'ah 87:3-4 requires leaving almonds visibly in almond milk when cooking fowl, even though fowl-and-milk is only a rabbinic prohibition, precisely because of mar'it ayin, and the same passage forbids cooking in human milk for the same reason; Shulchan Arukh, Yoreh De'ah 298:1-2 forbids silk-with-wool and kalakh-with-flax due to their resemblance to forbidden mixtures even though the underlying concern is appearance rather than the Torah's own shatnez; and Shulchan Arukh, Orach Chayim 301:11 and Mishnah Berurah 301:25 forbid ornamental items on Shabbat out of mar'it ayin even when the actual carrying would otherwise be permitted or only rabbinically regulated.
Shulchan Arukh, Orach Chayim 243:1-2 and Mishnah Berurah 243:1 apply mar'it ayin to the rental of a bathhouse to a non-Jew specifically because, although the non-Jew's Shabbat labor would not implicate the Jew under strict law, onlookers would suspect the non-Jew is working as the Jew's agent — the Mishnah Berurah states explicitly that the Sages prohibited this arrangement in the bathhouse case specifically because of mar'it ayin, distinguishing it from cases that are permitted outright by din.
Igros Moshe OC 3:50 directly addresses whether mar'it ayin applies to rabbinic prohibitions, noting that some claim it does not, but concluding based on the Shach and the Rema that it does apply even to rabbinic Shabbat prohibitions, while stressing that there is no mar'it ayin concern at all when the underlying act is genuinely permitted.
Darkhei Teshuvah, Yoreh Deah 99:19 and Darkhei Teshuvah, Yoreh Deah 99:82 likewise record the position, citing the Shach, that the law of mar'it ayin applies even to a rabbinic prohibition that has no Torah-level root at all, though they note that in a case prohibited solely because of mar'it ayin, one may even add to it לכתחילה precisely in order to nullify the forbidden mixture and remove the appearance of prohibition.
Sdei Chemed, Maarechet Kaf 28 refines the scope of the rule by citing the Malei HaRo'im's distinction: the decree of mar'it ayin applies even in innermost chambers only where the same act, performed in public, would actually be seen, whereas if in public one could avoid detection through some maneuver, the extension to private chambers does not apply.
Daat Kohen 9 (Rav Kook) holds that even though by strict Torah law a person may not forbid what does not belong to him, a rabbinic decree grounded in mar'it ayin can still take hold in such a case, though he limits this by excluding situations where it is known that the person's intent is merely to cause distress, in which case the Sages did not extend the decree.
Mishpat Kohen 33 (Rav Kook) rules that mar'it ayin must be treated as permitted at the outset even regarding what would otherwise be the principle that one should not add leniency to a rabbinic prohibition as though it were a Torah prohibition, indicating that the mar'it ayin concern itself operates with its own, more lenient, rules even within the realm of דרבנן.
Igros Moshe EH 1:35 goes further, reasoning that mar'it ayin is not simply a later-instituted rabbinic enactment but reflects a Torah-level concern for avoiding suspicion and chillul Hashem, implying that its force is not confined to, or weakened by, the rabbinic status of the underlying prohibition.
Tzitz Eliezer 13:15 treats a related application, discussing whether closing one's eyes suffices when facing exposed hair, and suggests that while this leniency is rejected regarding ervah, it may be sufficient when the underlying prohibition concerning hair is only rabbinic — again tying the strength of a mar'it ayin-type concern to the rabbinic status of the base prohibition.
Avnei Yashfeh 6:143 takes up a case where an act is itself rabbinically prohibited even absent mar'it ayin, concluding that in that scenario the mar'it ayin decree does not additionally apply in private, qualifying this based on whether a plausible permissive explanation exists and how common that permitted circumstance is.