First, it's a useful exercise to ask yourself *why* you want to
record in your markup the detail that the text was written up the
margin. It's unlikely that anyone wants to read a digital facsimile
that has text running up the side like that, and if you're going to
provide page images than anyone can easily see that's how the
original was written. Unless you imagine some particular digital
processing that requires it, then it may not be worth your time
and effort to record this.
That said, we would use something like
perhaps with some type= on the <seg> to differentiate it from other,
e.g. linguistic uses of, <seg>.
 E.g., you are encoding lots of documents that occasionally have
this feature, and you want to allow scholars to compare spelling
when the author was writing up the margin to her normal spelling.
> Working on a digital edition of mid-late 19th century letters, we
> encountered a dilemma and would be very thankful for some advice
> from the TEI-list:
> We would like to mark up instances where the text of a paragraph
> continues in the margin of the page. These are not "proper"
> additions (<add>), but have more to do with the appearance of the
> page. <seg> was being considered, but still leaves the problem of
> how to account for the placement of the text.
> example: <p>text text text until the bottom of the page<???>the
> text then continues in the margin of the page because the sheet is
> full and the writer is economic in the use of paper</???></p>