I think that the ONLY practical solution here, at the current stage of Scribus development,
where a piece of text in the header has to be dynamic in some way,
is to create multiple master pages. Then you apply these master pages as you see fit.
Here again I will mention that in my use of Scribus I don't keep all of the chapters of a
book in one file, but keep them in separate files which are manageable within my memory
limitations, so that Scribus does not run slowly. So, there are only a handful of master pages
for each file the way I work, and often just three, a right, a left, and a first-chapter-page.
But it would not be difficult to set up a number of master pages with more information included,
so that each section within a chapter has its own master page with its own header with the
section number.
So, for instance, if I have a chapter which, for clarity, ought to have running headers which reflect
the content of a particular section, or if I wish to put running section numbers at the tops of
pages, then the easy way is to make a lot of separate master pages with the running headers
hard coded, and apply them as desired.
However, there is a function in the Doc Setup where sections of the file may be defined. I have
used this area of the Setup only for setting the initial page number for the beginning of each
chapter. I have not experimented with the following possibility which appears to be implied as
an eventual fully developed capacity: define sections which could be tied to specific master pages
or possibly even to specific dynamic fields within one master page.
As I say, I don't think this is possible yet but it appears to be coming. If somebody has
experimented with this or if one of the development team knows that this is so, please comment.