Aidan Kehoe writes:
> I don’t think it ever has; #'read-multilingual-string in mule-cmds.el seems
> to be there exactly to work around this.
If somebody wants to look, the skk pacakge has its own isearch mode.
Perhaps that method can be adapted as a standard feature for regular
isearch. Also, GNU Emacs has been able to do this for a while, I
think. So there's probably a GPLv2 implementation, and of course for
personal use you're welcome to develop based on GPLv3 Emacs, and we
can integrate it when the GPLv3 conversion is done.