This is a further proposal for makeinfo-buffer,

	* textmodes/makeinfo.el (makeinfo-buffer): Look for TeX-master
	variable to indicate top-level texi file.

I've been using this with the Guile manual.  I know tex-main-file is
checked by tex-mode, I wasn't sure whether TeX-master or tex-main-file
or both should be used.  I went with TeX-master because that's what
Guile has currently.