The problem is that all of this is a neologism and, as such, there is no widespread understanding. Language evolves and in this case it is evolving quickly and in multiple directions.
For example, Wikipedia explicitly disagrees with what you wrote. Link: Preferred gender pronoun - Wikipedia and basically documents that actual usage is quite ambiguous and inconsistent.
That doesn’t make you wrong. That doesn’t make me wrong. But if the intention in setting the gender pronouns field is to a) communicate clearly and b) have people use the correct gender pronouns - then there needs to be that widespread understanding.
A diligent person could see “a/b/c” or “a/b” listed in the pronouns, not understand precisely what is meant, go to Wikipedia and come away with an incorrect understanding. Technically, I am being a bit pedantic because I doubt anyone could confuse a possessive pronoun with a nominative case pronoun. At least I am not aware (in English) that any possessive pronoun is also used as a nominative case pronoun - but you don’t want people to have to think too hard. Many people would not understand the grammatical distinctions being made anyway.
Hence my suggestion that this information be expressed using a single standard and unambiguously.
Another consideration for wanting this information to be expressed well is that sometimes the forum software itself uses third person pronouns. If you expect software to get it right (you may not but I do) then precision is key.
In the absence of an ISO standard, JCS will do his best and I hope all forum users will do their best.
I can’t speak for anyone who actually would want an option similar to that but, firstly, “all” may be better than “any/all” (just keeping the syntax correct) but, secondly, “any” or “all” may not express what is really going on. Does it mean that the person does not care? (In that case should the field have been left blank?) Does it mean that the person is fluid?
So I think that if any new forum user contacts JCS and requests something like that then the forum user would need to elaborate in the corresponding new introduction topic.
How could this be detected or enforced if there is no way for a forum user to express the pronouns that should be used?
Yes, this is true. You cannot please all of the people all of the time.
Minor correction: The private message request must go to @JCS. Not me.
I don’t see how this works.
Just because you use a name that does not distinguish “he” from “she” does not mean that you are choosing to express being non-binary. It might just be that you are, you know, using your actual name, the name that your parents gave you.
This may not be ideal for non-English speakers either. Putting the boot on the other foot, if someone has a name that is in a language that is quite different from English, how will I know what pronouns the person wants?
I believe also that a user cannot change the name retrospectively. Therefore if a user initially chooses to withhold gender information (in a privacy-focussed forum that kind of makes sense) but later on chooses to make that information publicly available, the forum software does not support that. Note that this applies to cis- situations as well as trans- situations as well as non-binary situations.