The pinentry.info (generated from pinentry.texi) is missing the info file name from the dirsection. That name needs to be present for other Info readers (eg. Emacs) to locate the right .info file. Attached is a patch to fix this.
Created attachment 86616 [details, diff] 0.7.2-info-dir-gentoo.patch sets the info file name for pinentry's dirsection
Added to pinentry-0.7.2-r3, thanks Matthew!