\define[1]\ChapterListNumber
{\simplealignedbox{\listparameter{width}}{flushright}{#1}}
\setuplist[chapter][width=2em,distance=1em,numbercommand=\ChapterListNumber]
This now gives a toc looking the way I like.
May I lay at your foot a plea for a change/additional parameter here? \setuplist[][itemalign=left/right] or something in that sense.
For numbered chapters I consider an alignment as below most natural, although opinions may differ.
When there would a key for this it should be named numberalign.
By the way, the \define[1] eludes me. Why this particular [1] there?
You specify the number of argument for the \define command with the number, e.g.
\define\command{…} is equal to \unexpanded\def\command{…}
and
\define[3]\command{…} is equal to \unexpanded\def#1#2#3{…}
When you want to create a command which is similar to \def (without \unexpanded) you can use \defineexpandable[<number>]\command
Why do you think lists use the itemgroup/itemize mechanism?
I didn’t think that exactly, but just tried it in despair and having the idea that toc items are set as an itemized list (it looks that way, isn’t it?).
No, they are unrelated but both mechanism use the same low level commands for the indentation of the text.
Wolfgang