After much searching, my probably laughable attempt consists of combining
the answer to the question at
https://tex.stackexchange.com/questions/433110/setting-page-attributes-of-every-page-in-a-generated-context-file
with
what I can see in the source at
https://source.contextgarden.net/tex/context/base/mkiv/lpdf-mis.lua:

\appendtoks
  \startluacode
      local formatters = string.formatters

      local pdfverbose = lpdf.verbose
      local pdfarray = lpdf.array

      local factor  = number.dimenfactors.bp
      local f_value = formatters["\letterpercent.6N"]

      local function boxvalue(n)
          return pdfverbose(f_value(factor * n))
      end

      lpdf.addtopageattributes("TrimBox", pdfarray {
          boxvalue(30),
          boxvalue(30),
          boxvalue(180),
          boxvalue(267),})
  \stopluacode
\to \aftereverypage

This code throws no errors, but unfortunately it also has no effect. What
am I doing wrong?

Jack

On Sat, 25 Apr 2020 at 22:37, Jack Steyn <steynjack@gmail.com> wrote:

> Hi,
>
> I am having trouble setting the PDF boxes to my desired dimensions.
> Suppose I have \setuppapersize[*a*][*b*]. I want CropBox = MediaBox = *b*.
> So far, so good: I can just use cropoffset=0mm in \setuplayout. But I want
> TrimBox = *a*. However, as far as I can see, I can't achieve this using
> trimoffset in \setuplayout unless (width of *b*) – (width of *a*) =
> (height of *b*) – (height of *a*), which does not hold in my case. So it
> looks like I need to find another way to set the TrimBox (and the BleedBox,
> which I want to be 3mm wider and taller than the TrimBox). Does anyone know
> how to do this?
>
> Best,
>
> Jack
>