During the Passover holiday, I managed to find myself in the middle of an argument as to whether adding an orange on the seder plate is "kosher." To provide a bit of background information on this custom, it started back in the 1980s by Susannah Heschel, a Jewish feminist scholar. Initially, Heschel wanted to but bread crumbs on the seder plate to communicate the notion "that there is as much room for a lesbian in Judaism as there is for bread crumbs on a seder plate." However, this would have caused problems because it would have violated Passover dietary restrictions. So instead, Heschel opted with an orange to symbolize the fruitfulness for all Jews when gay men and lesbians are free to be contributing members of Jewish society, as opposed to the oppression that comes with the current status of halacha (Jewish law), as well as what would be defined as the societal norms in [traditional] Jewish society (a.k.a. "heteronormativity") that puts homosexuals into a state of being that is akin to slavery. The orange has not only been a symbol for inclusivity of LGBT members of the Jewish community, but to keep in mind anybody else who is marginalized in a similar fashion. More on the custom can be read here.
In the midst of the argument, a traditionalist/Orthodox Jew, who happens to be a local, Jewish authority figure, said that "Having an orange on the seder plate makes as much sense as having a 'Vote for Scott Walker' flag on your seder plate. Making such a political statement is simply not halachic, and if you do it, it's as if you were no longer practicing Judaism." I'm going to set aside the irony of that statement for a moment, and going to ascertain whether such a practice is acceptable under Jewish law.
In Jewish law, adding an orange to the seder plate would have to be considered a custom (מנהג, minhag). Minhagim are practiced on multiple levels. Some are "universal" in nature, such as wearing the kippah. Others are practiced based on the sub-groups of origin, such as the Ashkenazi prohibition of kitniyot on Passover. There are certain minhagim that are practiced on the familial or even the individual level, such as the practice I developed of studying the character trait (מדה) of gratitude on Thanksgiving. The primary criterion for a halachically valid custom is that it does not violate Jewish law. Otherwise, it would be considered a "mistaken custom," or מנהג טעות.
The main objection of adding the orange to the seder plate [in the aforementioned argument] is that one cannot add to the seder [plate], because if you do, you are violating halacha. The word seder (סדר) literally means "order." In order for this claimed level of meticulousness to be true, not only would the seder plates have to have been homogenous throughout history, but also the seders themselves would have to be homogenous in nature. Another way of saying this is "If I end up finding historical deviations from what is considered a 'normal seder,' then the statement is false."
Let's start off with the karpas. Initially, celery was used, although many have been accustomed to using parsley as karpas. Furthermore, the Ashkenazi are accustomed to dipping karpas into salt water, whereas Sephardim dip their karpas into vinegar. There is a longer list of such variations, but it can be argued that variations do not take any of the actual steps of the seder out of its "well-defined chronological order." However, Passover traditions vary in more than in terms of the dishes themselves.
There are multiple Passover traditions that have been added to the seder itself. In North African Jewish communities, it is common to pass the seder plate over the heads of everyone at the table in a circular motion to represent the transition from slavery to freedom. Afghan Jews added the custom of whipping themselves with scallions to represent the slavedrivers' whips against the Israelites. Certain Polish Jews add a re-enactment of the crossing of the Red Sea. In certain Sephardic communities, it is customary for the seder leader to walk around the table three times and tap the heads of the guests as a way to bless the guests. There are other practices that are similar to these in terms of their uniqueness.
What do all of these practices have in common? They were additions to the tradition of the seder, and to be able to add such traditions is a well-established practice. To add an orange to the seder plate is anything but a violation of Jewish law. It is a 21st-century manifestation of the evolution in Jewish practice that has been occurring since time immemorial. For certain Jewish individuals, putting an orange on the seder plate is their way of making a connection between the Exodus of the past and making it relevant to the current state of [Jewish] society, particularly with the treatment of LGBT individuals. It is a reminder that because "we [Jews] were strangers in the land of Egypt," we shouldn't subject individuals to such marginalization. It is a message that I hope resonates so much that rather than treating individuals as "the other," we treat them as human beings.