The following solution which adds two additional keys (pageleft and pageright)
is cleaner because it doesn’t have to overload commands.


\unprotect

\unexpanded\def\startregisterpages
  {\begingroup
   \dostarttagged\t!registerpages\empty
   \useregisterstyleandcolor\c!pagestyle\c!pagecolor
   \registerparameter\c!pageleft}

\unexpanded\def\stopregisterpages
  {\registerparameter\c!pageright
   \dostoptagged
   \endgroup}

\protect

\setupregister
  [index]
  [n=1,
   alternative=A,
   distance=0pt,
   pageleft=\wordright\bgroup,
   pageright=\egroup]

\starttext
\CONTEXT\index{\ConTeXt}
\page
\LATEX\index{\LaTeX}
\page
\CONTEXT\index{\ConTeXt}
\page
\placeindex
\stoptext


Wolfgang
MF
10. Mai 2018 um 12:35
This is a more polite version, that lets you restore the original
macros.

\let\origstartregisterpages=\startregisterpages%
\let\origstopregisterpages=\stopregisterpages%
\def\setupregisterpagescommand#1{%
\unexpanded\def\startregisterpages{\origstartregisterpages#1\bgroup }
\unexpanded\def\stopregisterpages{\egroup\origstopregisterpages}}
\def\resetregisterpagescommand{%
\let\startregisterpages=\origstartregisterpages%
\let\stopregisterpages=\origstopregisterpages}

\setupregister[index][n=1,alternative=A]

\starttext
\CONTEXT\index{\ConTeXt}
\page
\LATEX\index{\LaTeX}
\page
\CONTEXT\index{\ConTeXt}
\page
\setupregisterpagescommand{\wordright}
\placeindex
\page
\resetregisterpagescommand
\placeindex
\stoptext

btw, Hans, are you going to add an option to \setupregister? :)
___________________________________________________________________________________
If your question is of interest to others as well, please add an entry to the Wiki!

maillist : ntg-context@ntg.nl / http://www.ntg.nl/mailman/listinfo/ntg-context
webpage : http://www.pragma-ade.nl / http://context.aanhet.net
archive : https://bitbucket.org/phg/context-mirror/commits/
wiki : http://contextgarden.net
___________________________________________________________________________________