I just read that the Rogatchover Gaon ruled that a Noachide can become a Ger Toshav on his own, without Rabbinical approval (which is otherwise impossible until the yovel is declared), by simply accepting the Sheva Bnei Noachide laws. Razin d'Orayta, p. 97, §33.
However, Chabad of all people seems to take an opposite approach to non-converts:
One may not invite a non-Jew to a Yom Tov meal unless Shabbat coincides with that Yom Tov. The reason for this is that one may inadvertently cook for the non-Jew on Yom Tov, which is forbidden. On Shabbat when one may not cook in any case, it is permitted to invite a non-Jew. If the non-Jew comes without being invited, one may feed him on a regular Yom Tov as well but may not cook or heat up food for him. There is no distinction between the Pesach Seder and other Yom Tov days in this regard. Orach Chaim 512:1, Shulchan Aruch HaRav ibid, 2.
I tried halachapedia and I consulted Divine Code but couldn't find anything. They all wrote the same thing, that a goy/Goya is prohibited to participate in holidays as they weren't commanded to, unless they were invited, they acted duplicitously, or it was Shabbat. Nothing about a Giyoret Toshavah or a Giyoret Tzedeket. Obviously, an aku"m can't make new holidays, and I can see why nachrim would be prohibited as they are actively refusing to get with the picture, maybe the Chasidei Umot Ha-olam or even the Chochmei Umot Ha-olam, as these people are technically stam goyim, but once a Noachidess goes the extra step as a Toshavah but doesn't want to convert to a full blown Tzedeket, don't they have enough of a yiddisher neshamah to participate in Pesach?