Thanks @ gcwarbler, I have been slowly working my way through all the linked discussion threads to this topic. Posting a link to a journal article in the “More Info” tab seems to be a workable option for curators to use. For the rest of us though, this method does not appear to be available.
As noted above and in other associated threads, Wikipedia is not a guide, and identification keys are not appropriate material to add there. What I can do however, is post a link (or at least the publication info) of an external reference that contains the identification key. When I found a useful key to identifying the species of the tribe Psenini, I posted the info as an external link on the Wikipedia article under the heading “Further Reading” (as Gittens, 1969). Unfortunately, in this case the article is behind a paywall and is not accessible to everyone.
If there is any iNaturalist user that would like me to help them to do something similar, I can certainly help, just send me a message with the details of what you would like to see added to a specific Wikipedia article.