See [http://www.mail-archive.com/ntg-context@ntg.nl/msg87812.html this thread]
on the mailing list.
In MkXL (lmtx) one would rather use:
<pre>
\setupregister[index]
[pageleft=\hfilll,
pageright=\frozen\parfillleftskip 0pt plus 1fill\frozen\parfillrightskip 0pt\par]
</pre>
== See also ==