\setupnotedefinition

[footnote]

[ location=left,

 headalign=flushright,

     width=1em,

  distance=1ex]

This comes very close to what I have in mind :-)

Is it possible to not fix the width globally but rather use the largest actual width encountered in any single footnote block? That is, a footnote block with only single digits would still align left with the textblock boundary as would a footnote block with two digit numbers only (I'm using tabular figures) and  just for mixed one and two digit blocks the right alignment would actually become visible…