I hope I'm not opening a can of worms by trying to figure out a best practice for our site.
It's also my understanding that this is the Meta sibling of Are there any non-binary pronouns or neopronouns in Spanish?, or at least one of them, since figuring out what we can really use (or want) may need a couple questions on the main site and figuring out how to come to agreement to use those items may take more than one Meta question.
My proposal would be, if you wanted to make an effort to deliberately address users in a gender neutral way in Spanish, to address them as "él/ella" (not "él" or "ella", but "él/ella"). Please note that the usage of a given pronoun doesn't fix the issue in Spanish, since you may need agreement in adjectives, nouns, articles, etc. For such adjectives I would default to masculine or try to use the "o/a" from.
él/ella es muy listo/a
él/ella está enfadado/a por ...
By using this complicated form (and I mean complicated, not complex) you convey that you are trying to avoid misgendering a user (and/or abiding the CoC rules of addressing them in a neutral form until you know their pronouns). I believe that it may be a different question if you need to go this far and be overly cautious with every single user for who you are unsure of their gender (but then, there is the new CoC and its FAQ, which may be creating more problems than the ones it's trying to solve...).
Additionally, you could follow the tips provided in ¿Es posible ocultar mi género en Internet al hablar en español? and avoid adjectives or nouns that need a gender and substitute them for synonyms that not necessarily imply a gender. Other similar tips are also suggested in Are there any non-binary pronouns or neopronouns in Spanish?
userName es muy inteligente
userName se enfadó por...
Another option would be to use the masculine until corrected. This may be counter-intuitive if, out of respect, you are trying to avoid misgendering (or if you are trying to play by the CoC), but masculine is the closest Spanish has for gender-neutral. If the user requests other pronouns you can simply apologize and explain that in their Nueva gramática de la lengua española (2009, sec. 2.2f) it is stated that
El "género no marcado" en español es el masculino, y el "género marcado" es el femenino.
and that you were just trying to use masculine as gender neutral, since Spanish has no specific gender for every single reality (such as transgender, hermaphrodite, etc.) and masculine seems to be the inclusive gender (as opposed to feminine, which seems to be the exclusive) and that the CoC requests that you address people in a gender-neutral way.
