This is a good-enough solution to my problem:
\setupalign[stretch]
\starttext
\def\Text{This is the text of an index term, that is normally justified,
but I'd like the page numbers to be flushed to the right from
their second line}
\def\Pages{34, 57, 101-104, 276, 345, 401, 403, 512.}
\type{\PlaceIndexTerm} has 6 parameters:
\startitemize[n]
\item the main text
\item the pages' numbers
\item the text width
\item the left margin of the lines made only of page numbers
\item the width of the separator (an hbox) before the first page number,
when there's room
in the last line of the text
\item the minimum accepted space left at the end of the last line of
text, to put a page number
\stopitemize
\def\PlaceIndexTerm#1#2#3#4#5#6{%
\newdimen\TextWidth \TextWidth=#3
\newdimen\SBPN \SBPN=#5 % space before page numbers
\newdimen\PagesIndentation \PagesIndentation=#4
\newdimen\MSLLL \MSLLL=#6 % minimum space left on last line
\parshape 1 0pt \TextWidth #1\par
\newdimen\LLW \LLW=\lastlinewidth
\page[no]
\ifdim \dimexpr\TextWidth-\LLW-\SBPN > \MSLLL
\blank[-\the\lineheight,fixed]\fi
\startalignment[flushright,stretch]
\ifdim \dimexpr\TextWidth-\LLW-\SBPN > \MSLLL
\parshape 2
\LLW \dimexpr \TextWidth - \LLW
\PagesIndentation \dimexpr \TextWidth - \PagesIndentation \red
\else
\parshape 1 \PagesIndentation \dimexpr \TextWidth - \PagesIndentation
\darkgreen
\fi
\noindent\ifdim \dimexpr\TextWidth-\LLW-\SBPN > \MSLLL\hbox to
\SBPN{}\fi #2\par
\stopalignment}
\def\PlaceIndexTermDescribed#1#2#3#4#5#6{%
\newdimen\TW\TW=#3
\noindent{\tfx width={\bf \PtToCm{\the\TW}},
pages' left margin (2nd line)={\bf #4},
hbox before pages={\bf #5},\\%
min space left on last line of text={\bf #6}}\par
\blank[halfline]
\PlaceIndexTerm{#1}{#2}{#3}{#4}{#5}{#6}
\blank}
\page
\PlaceIndexTermDescribed{\Text}{\Pages}{3cm}{12pt}{1em}{12pt}
In this case there's no space left on the last line of text,
so the page numbers are flushed to the right, starting from
a new line.
\page
\PlaceIndexTermDescribed{\Text}{\Pages}{3.5cm}{12pt}{1em}{12pt}
Now the last line of text is short and there's room to start the page
numbers on the same line. The remainder of page numbers is flushed right,
with a (min) left margin of 12pt.\par
Actually, the page numbers are started in a new paragraph with a
\type{\parshape}
that depends on the last line of the text; then the new paragraph is moved
upwards with a \type{\blank[-\the\lineheight,fixed]}.
A \type{\page[no]} before the \type{\blank} guarantees that
no page break occurs between text and numbers.
\page
\PlaceIndexTermDescribed{\Text}{\Pages}{4cm}{12pt}{1em}{12pt}
\page
\PlaceIndexTermDescribed{\Text}{\Pages}{4.5cm}{12pt}{1em}{12pt}
\page
\PlaceIndexTermDescribed{\Text}{\Pages}{5cm}{12pt}{1em}{12pt}
\page
\PlaceIndexTermDescribed{\Text}{\Pages}{5.5cm}{12pt}{1em}{12pt}
\page
\PlaceIndexTermDescribed{\Text}{\Pages}{6cm}{12pt}{1em}{12pt}
\page
\PlaceIndexTermDescribed{\Text}{\Pages}{6.5cm}{12pt}{1em}{12pt}
\page
\PlaceIndexTermDescribed{\Text}{\Pages}{7cm}{12pt}{1em}{12pt}
\page
\PlaceIndexTermDescribed{\Text}{\Pages}{7.5cm}{12pt}{1em}{12pt}
\page
\PlaceIndexTermDescribed{\Text}{\Pages}{8cm}{12pt}{1em}{12pt}
That \type{512} left on the last line is ugly, and we can trim the
separator hbox
from \type{1em} to \type{.1em} to let all the page numbers fit in the
last line:\par
\blank
\PlaceIndexTermDescribed{\Text}{\Pages}{8cm}{12pt}{.1em}{12pt}
or we could increase the left margin of the lines made only of page numbers
(to \type{3cm} in this case), to distribute the numbers better across
the two lines:\par
\blank
\PlaceIndexTermDescribed{\Text}{\Pages}{8cm}{3cm}{1em}{12pt}
\page
\PlaceIndexTermDescribed{\Text}{\Pages}{8.5cm}{12pt}{1em}{12pt}
\page
\PlaceIndexTermDescribed{\Text}{\Pages}{9cm}{12pt}{1em}{12pt}
\page
\PlaceIndexTermDescribed{\Text}{\Pages}{9.5cm}{12pt}{1em}{12pt}
Increasing the left margin of the lines of page numbers gives a better
result:\par
\blank
\PlaceIndexTermDescribed{\Text}{\Pages}{9.5cm}{4cm}{1em}{12pt}
\page
Full text width (third parameter set to \type{\textwidth}):\par
\blank
\PlaceIndexTermDescribed{\Text}{\Pages}{\textwidth}{12pt}{1em}{12pt}
\stoptext
(Thanks again to Wolfgang for answering my question)
Massi
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