Since about two years ago, there has been a "srvdir" USE flag in Gentoo that is supposed to direct packages like Apache, ProFTPd, and other daemons that store or access data for services offered by the system. Despite this flag, very few packages use the /srv directory, even when the "srvdir" flag is turned on and /srv has been created. In reality, most packages that store data for services don't support the USE flag. A quick `equery hasuse -p srvdir' shows that only mysql uses it. This bug is a meta bug, which will depend on bugs for individual packages that users would like to support srvdir.
Honestly, you'd have better luck with asking at gentoo-dev mailing list, otherwise this won't move very far... ;)
Both authors of GLEP20 retired. Marking this LATER.