"There's an open issue to allow callouts to be used in prose. See
https://github.com/asciidoctor/asciidoctor/issues/3037"
If I'm reading that right, what's being requested in that issue is callouts in the target (first use) in arbitrary places, not callouts in the annotation text in arbitrary places.
I still don't see anything in the text here that actually constrains where the callouts in the annotation appear - all it says is "The second use, which goes below the verbatim block, defines the annotation text."