The ebuild for texinfo does not actually install the info pages for itself. It ends up installing the info pages to the temporary image directory. I'm not familiar with how info pages are installed in ebuilds, so I can't help on this one.
Found out what happens. During install, the package gets installed into $TMP/portage/texinfo-N/image/$TMP/portage/texinfo-N/image/usr/share (The path is doubled) So when the ebuild tries to rename the info files, it fails because they're not where it expects. I'll investigate further.
This fixed it for me. Don't know if it's the Right Thing(tm). The makefile appears to append infodir to DESTDIR. 40c40 < make DESTDIR=${D} infodir=${D}/usr/share/info install || die --- > make DESTDIR=${D} infodir=/usr/share/info install || die
Thanks for the catch, AND the fix, Vadim :)