Hi Jano,
It's not clear to me what you want to measure when but here is a weekend
playground (you're locked down anyway).
\def\MacroJano#1%
{\writestatus{JANO}{ONE}
\setbox0=\hbox\bgroup\xmlflush{#1}\egroup}
\startluacode
function document.FunctionJano(id)
context.writestatus("JANO","ZERO")
token.expand_macro("MacroJano",true,id)
context("%p",tex.box[0].width)
context.writestatus("JANO","TWO")
end
\stopluacode
\startbuffer[doc]
<?xml version "1.0"?>
<document>
<section>one</section>
<section>one two</section>
<section>one two three</section>
</document>
\stopbuffer
\startxmlsetups xml:name
\xmlsetsetup{#1}{*}{xml:*}
\stopxmlsetups
\xmlregistersetup{xml:name}
\startxmlsetups xml:section
width: \ctxlua{document.FunctionJano("#1")}\par
\stopxmlsetups
\startxmlsetups xml:document
\xmlflush{#1}
\stopxmlsetups
\starttext
\xmlprocessbuffer{xml:name}{doc}{}
\stoptext
A clue: token.expand_macro will kind of 'run a local tex job'.
If this works for you. you know the price you have to pay ... wikify
Hans
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