Thank you a lot, Hans. Indeed, the second solution seems to correctly put
text into a vertical flow. However, a serious issue with Mongolian script
is the horizontal direction: unlike CJK vertical typesetting, the text
columns go from left to right, and the following almost works (\definefont
gives no ligatures in my computer so I've used \definefontfamily instead)

%\definefont does not work as expected
\definefontfamily[main][serif][mongolianwhite][features=mongolian]
\setupbodyfont[main,24pt]
\defineframed
[vertical]
[
offset=none,
align=flushleft,
anchoring=left,
foregroundstyle={\setupinterlinespace[40pt]}]

\starttext
%\MyFont
\startframed[vertical]
ᠣᠢᠷ᠎ᠠ ᠵᠢᠨ ᠡᠳᠦᠷ᠂ ᠦᠪᠦᠷ ᠮᠣᠩᠭᠣᠯ ᠦᠨ ᠰᠢᠯᠢ ᠵᠢᠨ ᠭᠣᠣᠯ ᠠᠢᠮᠠᠭ ᠦᠨ ᠰᠢᠯᠣᠭᠣᠨ ᠬᠦᠬᠡ
ᠬᠣᠰᠢᠭᠣᠨ ᠦ ᠰᠢᠭᠦᠬᠦ ᠬᠣᠷᠢᠶ᠎ᠠ ᠨᠢᠭᠡᠨ ᠬᠦᠴᠦᠷᠬᠡᠭᠯᠡᠨ ᠨᠦᠵᠢᠳᠯᠡᠭᠰᠡᠨ ᠶᠠᠯᠠᠲᠣ ᠬᠡᠷᠡᠭ ᠲᠦ
ᠰᠢᠭᠦᠯᠲᠡ ᠬᠢᠪᠡ᠃ ᠰᠢᠭᠦᠨ ᠲᠠᠰᠣᠯᠣᠯᠳᠠ ᠪᠠᠷ᠂ ᠵᠠᠭᠠᠯᠳᠣᠭᠳᠠᠭᠴᠢ ᠡᠷᠬᠢᠮᠲᠦ ᠵᠢ ᠬᠦᠴᠦᠷᠭᠡᠭᠯᠡᠨ᠂
ᠨᠦᠵᠢᠳᠯᠡᠭᠰᠡᠨ ᠶᠡᠯ᠎ᠡ ᠪᠡᠷ ᠭᠣᠷᠪᠠᠨ ᠵᠢᠯ ᠦᠨ ᠬᠣᠭᠣᠴᠠᠭ᠎ᠠ ᠲᠠᠢ ᠬᠣᠷᠢᠬᠣ ᠡᠷᠡᠭᠦᠦ ᠪᠡᠷ
ᠰᠢᠳᠬᠡᠭᠰᠡᠨ ᠪᠠᠢᠨ᠎ᠠ᠃
\stopframed
\stoptext

I've attached two files: the ConTeXt output (results so far) and the
expected result (done via LuaLaTeX). As it can be seen, the text lines
should go "upwards". I'd just use LuaLaTeX for this specific case, but
ConTeXt is too cute...

Thank you again, Hans.

Sincerely,
Jairo.

El mar., 4 de ago. de 2020 a la(s) 07:52, Hans Hagen (j.hagen@xs4all.nl)
escribió:

> On 8/4/2020 2:42 PM, Hans Hagen wrote:
> > On 8/4/2020 10:32 AM, Jairo A. del Rio wrote:
> >
> >> http://www.mongolfont.com/en/font/mnglwhiteotf.html
>
> let me be fast in providing an easier solution (I bet WS would correct
> me otherwise):
>
> \defineframed
>    [vertical]
>    [offset=none,
>     align=flushleft,
>     anchoring=left,
>     foregroundstyle={\MyFont\setupinterlinespace[40pt]}]
>
> \startframed[vertical]
>      ᠣᠢᠷ᠎ᠠ ᠵᠢᠨ ᠡᠳᠦᠷ᠂ ᠦᠪᠦᠷ ᠮᠣᠩᠭᠣᠯ ᠦᠨ ᠰᠢᠯᠢ ᠵᠢᠨ ᠭᠣᠣᠯ ᠠᠢᠮᠠᠭ ᠦᠨ ᠰᠢᠯᠣᠭᠣᠨ ᠬᠦᠬᠡ
> ᠬᠣᠰᠢᠭᠣᠨ ᠦ ᠰᠢᠭᠦᠬᠦ ᠬᠣᠷᠢᠶ᠎ᠠ ᠨᠢᠭᠡᠨ ᠬᠦᠴᠦᠷᠬᠡᠭᠯᠡᠨ ᠨᠦᠵᠢᠳᠯᠡᠭᠰᠡᠨ ᠶᠠᠯᠠᠲᠣ ᠬᠡᠷᠡᠭ ᠲᠦ
> ᠰᠢᠭᠦᠯᠲᠡ ᠬᠢᠪᠡ᠃ ᠰᠢᠭᠦᠨ ᠲᠠᠰᠣᠯᠣᠯᠳᠠ ᠪᠠᠷ᠂ ᠵᠠᠭᠠᠯᠳᠣᠭᠳᠠᠭᠴᠢ ᠡᠷᠬᠢᠮᠲᠦ ᠵᠢ ᠬᠦᠴᠦᠷᠭᠡᠭᠯᠡᠨ᠂
> ᠨᠦᠵᠢᠳᠯᠡᠭᠰᠡᠨ ᠶᠡᠯ᠎ᠡ ᠪᠡᠷ ᠭᠣᠷᠪᠠᠨ ᠵᠢᠯ ᠦᠨ ᠬᠣᠭᠣᠴᠠᠭ᠎ᠠ ᠲᠠᠢ ᠬᠣᠷᠢᠬᠣ ᠡᠷᠡᠭᠦᠦ ᠪᠡᠷ
> ᠰᠢᠳᠬᠡᠭᠰᠡᠨ ᠪᠠᠢᠨ᠎ᠠ᠃
> \stopframed
>
> I realized that I already added the anchoring features long ago ...
>
> Hans
>
> > hm, only for testing, not for use
> >
> >> I can rotate boxes in ConTeXt using other methods than those used in
> >> LaTeX. However, my main concern is about "translating" the Lua(La)TeX
> >> piece to be used in ConTeXt. I've read about "finalizers" in the CLD
> >> manual and somewhere online, but I don't know how to access to
> >> node.subtype("pdf_literal"). I'll be happy to know about a better
> >> solution if ConTeXt offers it so it is accessible to other users.
> >> Thank you in advance.
> > The problem (or challenge) as usual is not in the coding but in what is
> > a reasonable user interface. So, what are the use cases. Her eis one
> > solution:
> >
> > \protected\def\startvertical
> >    {\dosingleempty\dostartvertical}
> >
> > \def\dostartvertical[#1]%
> >    {\begingroup
> >     \getdummyparameters[width=\textwidth,height=\textheight,#1]%
> >     \vbox
> >       orientation 1
> >       \doifelsedimension{\dummyparameter{width}}{to
> > \dummyparameter{width}}{}%
> >     \bgroup
> >       \doifelsedimension{\dummyparameter{height}}{\hsize
> > \dummyparameter{height}}{}}
> >
> > \protected\def\stopvertical
> >    {\vfill
> >     \egroup
> >     \endgroup}
> >
> > % test
> >
> > \starttext
> >
> > \definefont
> >    [MyFont]
> >    [file:mnglwhiteotf.ttf*default,mongolian @ 24pt]
> >
> > \showframe
> >
> > \startvertical
> >      \MyFont
> >      \setupalign[stretch,tolerant,stretch]
> >      \setupinterlinespace[40pt]%
> >      ᠣᠢᠷ᠎ᠠ ᠵᠢᠨ ᠡᠳᠦᠷ᠂ ᠦᠪᠦᠷ ᠮᠣᠩᠭᠣᠯ ᠦᠨ ᠰᠢᠯᠢ ᠵᠢᠨ ᠭᠣᠣᠯ ᠠᠢᠮᠠᠭ ᠦᠨ ᠰᠢᠯᠣᠭᠣᠨ ᠬᠦᠬᠡ
> > ᠬᠣᠰᠢᠭᠣᠨ ᠦ ᠰᠢᠭᠦᠬᠦ ᠬᠣᠷᠢᠶ᠎ᠠ ᠨᠢᠭᠡᠨ ᠬᠦᠴᠦᠷᠬᠡᠭᠯᠡᠨ ᠨᠦᠵᠢᠳᠯᠡᠭᠰᠡᠨ ᠶᠠᠯᠠᠲᠣ ᠬᠡᠷᠡᠭ ᠲᠦ
> > ᠰᠢᠭᠦᠯᠲᠡ ᠬᠢᠪᠡ᠃ ᠰᠢᠭᠦᠨ ᠲᠠᠰᠣᠯᠣᠯᠳᠠ ᠪᠠᠷ᠂ ᠵᠠᠭᠠᠯᠳᠣᠭᠳᠠᠭᠴᠢ ᠡᠷᠬᠢᠮᠲᠦ ᠵᠢ ᠬᠦᠴᠦᠷᠭᠡᠭᠯᠡᠨ᠂
> > ᠨᠦᠵᠢᠳᠯᠡᠭᠰᠡᠨ ᠶᠡᠯ᠎ᠡ ᠪᠡᠷ ᠭᠣᠷᠪᠠᠨ ᠵᠢᠯ ᠦᠨ ᠬᠣᠭᠣᠴᠠᠭ᠎ᠠ ᠲᠠᠢ ᠬᠣᠷᠢᠬᠣ ᠡᠷᠡᠭᠦᠦ ᠪᠡᠷ
> > ᠰᠢᠳᠬᠡᠭᠰᠡᠨ ᠪᠠᠢᠨ᠎ᠠ᠃
> > \stopvertical
> >
> > \page
> >
> > \startvertical[height=.75\textheight]
> >      \MyFont
> >      \setupalign[flushleft]
> >      \setupinterlinespace[40pt]%
> >      ᠣᠢᠷ᠎ᠠ ᠵᠢᠨ ᠡᠳᠦᠷ᠂ ᠦᠪᠦᠷ ᠮᠣᠩᠭᠣᠯ ᠦᠨ ᠰᠢᠯᠢ ᠵᠢᠨ ᠭᠣᠣᠯ ᠠᠢᠮᠠᠭ ᠦᠨ ᠰᠢᠯᠣᠭᠣᠨ ᠬᠦᠬᠡ
> > ᠬᠣᠰᠢᠭᠣᠨ ᠦ ᠰᠢᠭᠦᠬᠦ ᠬᠣᠷᠢᠶ᠎ᠠ ᠨᠢᠭᠡᠨ ᠬᠦᠴᠦᠷᠬᠡᠭᠯᠡᠨ ᠨᠦᠵᠢᠳᠯᠡᠭᠰᠡᠨ ᠶᠠᠯᠠᠲᠣ ᠬᠡᠷᠡᠭ ᠲᠦ
> > ᠰᠢᠭᠦᠯᠲᠡ ᠬᠢᠪᠡ᠃ ᠰᠢᠭᠦᠨ ᠲᠠᠰᠣᠯᠣᠯᠳᠠ ᠪᠠᠷ᠂ ᠵᠠᠭᠠᠯᠳᠣᠭᠳᠠᠭᠴᠢ ᠡᠷᠬᠢᠮᠲᠦ ᠵᠢ ᠬᠦᠴᠦᠷᠭᠡᠭᠯᠡᠨ᠂
> > ᠨᠦᠵᠢᠳᠯᠡᠭᠰᠡᠨ ᠶᠡᠯ᠎ᠡ ᠪᠡᠷ ᠭᠣᠷᠪᠠᠨ ᠵᠢᠯ ᠦᠨ ᠬᠣᠭᠣᠴᠠᠭ᠎ᠠ ᠲᠠᠢ ᠬᠣᠷᠢᠬᠣ ᠡᠷᠡᠭᠦᠦ ᠪᠡᠷ
> > ᠰᠢᠳᠬᠡᠭᠰᠡᠨ ᠪᠠᠢᠨ᠎ᠠ᠃
> > \stopvertical
> >
> > \stoptext
> >
> > But it's kind of a quick hack interface. We can have alignment,
> > interlinespace etc parameters to. Maybe definable verticals and so.
> >
> > Hans
> >
> > ps. for chinese one would probably kick in an extra feature
> >
> > % \definefontfeature
> > %   [vertical]
> > %   [vertical={%
> > %     orientation=3,%
> > %     down=.1,%
> > %     right=.1,%
> > %     ranges={%
> > %       cjkcompatibility,%
> > %       cjkcompatibilityforms,%
> > %       cjkcompatibilityideographs,%
> > %       cjkcompatibilityideographssupplement,%
> > %       cjkradicalssupplement,%
> > %     % cjkstrokes,%
> > %       cjksymbolsandpunctuation,%
> > %       cjkunifiedideographs,%
> > %       cjkunifiedideographsextensiona,%
> > %       cjkunifiedideographsextensionb,%
> > %       cjkunifiedideographsextensionc,%
> > %       cjkunifiedideographsextensiond,%
> > %       cjkunifiedideographsextensione,%
> > %       cjkunifiedideographsextensionf,%
> > %     }%
> > %   }]
> >
> > and maybe some extreme hz would work out well too.
> >
> > -----------------------------------------------------------------
> >                                            Hans Hagen | PRAGMA ADE
> >                Ridderstraat 27 | 8061 GH Hasselt | The Netherlands
> >         tel: 038 477 53 69 | www.pragma-ade.nl | www.pragma-pod.nl
> > -----------------------------------------------------------------
> >
> ___________________________________________________________________________________
>
> >
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> >
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> >
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>
> --
>
> -----------------------------------------------------------------
>                                            Hans Hagen | PRAGMA ADE
>                Ridderstraat 27 | 8061 GH Hasselt | The Netherlands
>         tel: 038 477 53 69 | www.pragma-ade.nl | www.pragma-pod.nl
> -----------------------------------------------------------------
>