Continuation of bug 282482. I'll attach patches to update gst-plugins-* eclasses to further EAPI. As for prefix support, I'm not sure what needs to be done here. Mart could you elaborate, please? Also keep in mind that portage (in tree) does not support prefix at the moment so in any case it's prefix herd responsibility to check that everything done correctly and keyword ebuilds. At the same time we need EAPI=3 to be able use EAPI=4 later (and we don't need to check prefix support for that).
Created attachment 242789 [details, diff] gst-plugins-good.eclass.patch
Created attachment 242791 [details, diff] gst-plugins-bad.eclass.patch
Created attachment 242793 [details, diff] gst-plugins-ugly.eclass.patch
BTW, may be it's good idea to update eclasses and state explicitly that they are maintained by gstreamer herd as currently they state that gnome maintains them.
I think EAPI-3 support is mostly reviewing what places need $E* variables to be used instead of the versions without E prefix
New eclasses are currently being reviewed on gentoo-dev ml. Please check there.
New eclasses have been commited. Feel free to open a new bug report if you still have problems with these.