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 <mailto:massifr@fastwebnet.it>
> 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? :)
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