This bug tracks the implementation of the EAPI 4 feature mentioned in the summary. A patch is included in bug 303919 (attachment 222241 [details, diff]).
Thanks, this is in git now: http://git.overlays.gentoo.org/gitweb/?p=proj/portage.git;a=commit;h=978994aec411b0a935624d3da10ba0700d603e13