David, I am well aware of Texinfo's deficiencies wrt indexing. There is no way, at present, within Texinfo to do what you want. I can only suggest post-processing the relevant files with a script. Sorry.
BTW, the C texindex program that sorts indexes for texinfo.tex knows nothing of multibyte locales, unless I've totally missed something. There is no occurrence of mbs or strcoll in the source (it is C makeinfo which did comparisons using those), AFAIK texindex just compares bytes. It is essentially unchanged since rms wrote it ca.1987, including worrying about sorting data sets that don't fit in "core" ... karl
