Do any rabbis comment on how to relate to members of the subset of Chabad who believe that the Rebbe Menachem Mendel Schneerson zt"l was the Messiah? For example, do non-messianist Orthodox rabbis generally advise that people interact with messianists in some particular way, or do they generally instruct people to interact with messianists in the same way that they interact with any other Jew?
Rabbi Hershel Welcher was asked this question, and he referenced a quote from one of Rambam's letters about someone who was believed to be the messiah, then died; "some were crazy enough to think he was still the messiah after he died." Rabbi Welcher thus ruled that someone who believes a dead man is the messiah is not idolatrous nor an apostate -- he's just a little crazy. (I would assume that because the craziness is localized to one subject of belief and doesn't affect general behavior, we wouldn't call them a shoteh, halachically insane.)
Rabbi Yehuda Herzl Henkin writes similarly Bnei Banim 4:26: "Someone who is mistaken in the identify of the Moshiach is no different than one who thinks Moshiach must have red hair or weigh a certain amount, for all these are vanity and folly (הבל ושטות) but are not heresy."
I also know a pulpit rabbi who wrote to several notable poskim (whose names I won't mention as they may not have wished these letters to be published) who ruled similarly -- "it's wrong but it doesn't affect their halachic status per se", with different degrees of attitude. (See caveats below).
As far as "how wrong" is such a position, it's debatable, some rabbis would say eh, they're following a minority opinion in the Talmud that was overruled; and besides, some Talmudic figures saw messianic qualities in their own teachers. Some would say it's more wrong than that.
But the bigger question, as I understand it, is "how dangerous" is such an opinion? Many are seriously concerned about a slide from "he's coming back as the messiah" to "he nullified himself to G-d so much that you can bow to a picture of him" or the like. (For instance, if someone today follows Rabbi Eliezer's opinion, that a Mohel can drive on shabbos if necessary to perform a circumcision -- they're very clearly wrong, that debate was settled 1800 years ago. But there's not much theological or sociological danger of morphing into something that's not Judaism!)
How many Lubavitchers believe exactly what, and what the best approach is to help as many as possible stay within the boundaries of our Thirteen Principles and the Rambam's Laws of the Knowledge [of G-d] ... well those are thorny issues that people debate.
Rabbi Aharon Feldman, Rosh Yeshiva of Ner Israel Rabbinical College, wrote a responsum regarding how to interact with Meshichistim (defined by him as anyone who believes that R Menachem Mendel Schneerson Zichrono Livracha will be resurrected to become the promised Messiah). He writes that they are not considered heretics, and thus their testimony in religious court and their ritual slaughter is valid and they can even count for a Minyan. However, he writes that since their belief is so dangerously wrong, it is forbidden to assist them in publicizing this belief. If one is present when they overtly declare it (such as through the Yechi statement) then one must protest if possible or at least leave the room. Additionally, no one with this belief should be relied upon for religious rulings (psak) as they are certainly lacking in השגה נכונה proper reasoning, and they should not be appointed Rabbis or religious leaders. One should not go to hear them give divrei Torah as their words should be assumed to be in error, and even going to listen can cause others to mistakenly think the wrong beliefs are acceptable.
Based on my comment on the question the the questioner's reply:
... then I suppose the following should suffice for an answer:
[A letter by Rabbi Ahron Soloveichik (published in the Jewish Press); bold font is my own for this answer]
Note: I am aware that R. Soloveichik himself disagreed with this belief, but nonetheless - his message is still very clear.