Perhaps if the current locale is en_US (and
other en variants, since
many use the US layout and have
little need for input methods),
then
C-\
should not automatically select an input method the
first time it is
hit. Then the user will be prompted and
can hit C-g. This will overcome
the problem that if it is
hit by accident by someone who knows nothing
about input
methods, they have no way of knowing what has gone wrong.
That sounds
like a good approach, provided the initial prompt were only
asked the first
time (and the setting remembered persistently, via
Customize).
But
again, I can't speak for people who use this. Would en_US users who use
`C-\'
be bothered by an initial prompt?