Use Layman:
yes | layman -a linux-be
Set the required useflags and accept the keywords:
installed_overlay="$(portageq get_repo_path / linux-be)"
mkdir -p /etc/portage/package.use
ln -s "$installed_overlay"/Documentation/package.use/linux-be.use /etc/portage/package.use/
mkdir -p /etc/portage/package.accept_keywords
ln -s "$installed_overlay"/Documentation/package.accept_keywords/linux-be.keywords /etc/portage/package.accept_keywords/
emerge -avt {beadm,grub,genkernel,bemerge}::linux-be
See Installing linux-be.