Hans,

That works for the over-simplified case here, but fails in practice. For a \startnarrower[left] paragraph, the mark is still in the main margin, not the 'margin' of the narrowed paragraph. Also, the placement of the text in the margin is wrong and not easily controlled as with \llap and \rlap. The code started out more like:

\define\Mark{\color[middlegray]{\hskip.6cm\itb¿\ }}
\setwidthof{\Mark}\to\MarkWidth
...
\starttexdefinition stopBufTest
  \startluacode
      buffers.prepend("TestBuffer","\\dontleavehmode\\llap{\\Mark}")
  \stopluacode
  \setupnarrower[left=\MarkWidth]
  \startnarrower[left,right]
  \startparagraph
    \inlinebuffer[TestBuffer]
  \stopparagraph
  \stopnarrower
  \egroup
\stoptexdefinition