Drat, I thought I had already done this, but I only introduced the necessary functions in core (c0f87ff9c7a5) and forgot to call them in the editors.
After a temporary account is created, the wikitext editor shows a "Temporary user" popup message alongside the usual "Your edit was published" post-edit notification:
Other editors should do it as well.
(They probably shouldn't overlap like that – filed as T344878)