In portage-18.104.22.168 and 2.2.0_alpha124 there’s support for EAPI 5_pre1, which implements all of the features that are currently in the eapi-5 branch of PMS (including the features from EAPI 4-slot-abi, which I’ve blogged about before). For additional references about the upcoming EAPI 5, see the “EAPI 5 tentative features” wiki page.
If you’d like to experiment with EAPI 5_pre1, then you can refer to the corresponding portage documentation, and you may need to pay special attention to the new “Profile IUSE Injection” feature. Since the profiles currently aren’t configured for this feature yet, you’ll have to configure these variables yourself if your experimental ebuilds reference special flags (like x86, kernel_linux, elibc_glibc, and userland_GNU) without listing them explicitly in IUSE. Here’s an abbreviated example of what the variables should look like, which you can put in make.conf:
USE_EXPAND="ELIBC KERNEL USERLAND"
USE_EXPAND_IMPLICIT="ARCH ELIBC KERNEL USERLAND"
USE_EXPAND_VALUES_ARCH="amd64 ppc ppc64 x86 x86-fbsd x86-solaris"
USE_EXPAND_VALUES_KERNEL="FreeBSD linux SunOS"
I have not populated all of the above variables exhaustively, but these values should be enough to get you started. If you need a more complete set of ARCH values to list in USE_EXPAND_VALUES_ARCH, then you can grab the exhaustive set of values from arch.list.