As Mark H would have said to me "Read The Manual" (except that is not quite what he would have said!).

Using the -xpp-pdf-annotation: "type={note} text={hello world}"; does the trick fantastically (and will also output the contents of an attribute instead of set text - I am assuming this is not possible in item formats though!

[edited version as per Jonathan's remarks]

Indeed creating notes is simple.

And if you are using classic IF/XMLmode, remember that you can add a CSS stylesheet that contains the rules with the pdf css properties, like the -xpp-pdf-annotion one. When you name that CSS file according to the style you use for that job, you just add the -pdfba option when printing. Direct PDF will use that CSS file and produce a PDF with annotations

In the PDF Support in XPP manual you find the following on page 8-2 [PDF Tagging Features]:

CSS is used to tag the textual content and generate bookmarks/annotations
(links). Output options allow for creating a fully PDF/UA-compliant
document or using CSS to generate bookmarks and annotations only,
without having to tag all the content.
These properties work with both CSS-XML and XML Divisions, provided
that the XML is well-formed and the markup is based on the logical reading
order of the document.

PS: Mark H. was right, he was always right (even when he was wrong)