Ok, I've found a way, see below.
But maybe someone has some nice lua (or other) tricks to share... (I'm not sure accessing t.dt[1] is particularly safe when operating on nested structures.)

Best,
Denis

\startbuffer[test]
<?xml version="1.0" encoding="UTF-8"?>
<doc>
<chapter>
    <title>A short title</title>
    <p>asdf asdf asdf jklö jklö jklö</p>
  </chapter>
  <chapter>
    <title>A longer title bla bla bla bla bla bla bla bla</title>
    <p>
    asdf asdf asdf jklö jklö jklö
    </p>
  </chapter>
</doc>
\stopbuffer

\startxmlsetups xml:test
                \xmlsetsetup{#1}{*}{-}
                \xmlsetsetup{#1}{doc|chapter|p}{xml:*}
\stopxmlsetups

\xmlregisterdocumentsetup{test}{xml:test}

\startxmlsetups xml:doc
                               \xmlflush{#1}
\stopxmlsetups

\setuphead[chapter][expansion=yes] % make sure the stuff gets expanded on time
\startxmlsetups xml:chapter
                \startchapter
                               [
                                               title={\xmlfilter{#1}{/title/command(xml:chapter:title)}},
                                               marking={\xmlfilter{#1}{/title/command(xml:chapter:title:marking)}},
                               ]
                               \xmlflush{#1}
                \stopchapter
\stopxmlsetups

\startxmlsetups xml:p
                \xmlflush{#1}\par
\stopxmlsetups

\startluacode
                function xml.functions.getMarking(t)
                _,n = t.dt[1]:gsub("%S+","")
                if n > 10 then
                  local words = {}
                  for word in t.dt[1]:gmatch("%S+") do table.insert(words, word) end
                  local resWords = {}
                  resWords =  { unpack( words, 1, 7 ) }
                  res = table.concat(resWords, " ") .. " ..."
                else
                  res = t.dt[1]
                end
                context(res)
                end
\stopluacode

\startxmlsetups xml:chapter:title:marking
                \xmlfunction{#1}{getMarking}
\stopxmlsetups

\startxmlsetups xml:chapter:title
                \xmlflush{#1}
\stopxmlsetups


\setupheadertexts[chapter]

\starttext

\xmlprocessbuffer{test}{test}{}

\stoptext


Von: denis.maier@unibe.ch <denis.maier@unibe.ch>
Gesendet: Montag, 11. September 2023 16:47
An: ntg-context@ntg.nl
Betreff: [NTG-context] XML: flush to lua function

Hi,
I have the following xml setup with a lua function:

\startluacode
                function markingLength(s)
                _,n = s:gsub("%S+","")
                if n > 3 then
                context("yes" .. " ...") else context("no") end
                end
\stopluacode

\startxmlsetups xml:ink:ink-title:marking
                %\ctxlua{markingLength(\xmlflush{#1})}  % does not work
                \ctxlua{markingLength{"1 2 3 4"}}
\stopxmlsetups

While the function works, I cannot use xmlflush here. How can I properly pass the xml content to the lua function?

Best,
Denis