Since there was no reply on the question https://forums.gentoo.org/viewtopic-t-1069022-highlight-.html I suppose that there is currently no corresponding support in portage/pms: The problem is that for a public overlay (which by its nature is probably used with various profiles) there currently does not seem to be any way to introduce an ebuild which has USE-flags which need to be masked on certain architectures. Assume, for instance, package cat/foo is ARCH="~amd64 ~x86", and in an overlay, there is a package cat/bar with optional support for package cat/foo with ARCH="~amd64 ~x86 ~sparc". What would be done in the gentoo repository would be to introduce a USE-flag "bar" in cat/foo, to have in cat/foo a dependency of the form RDEPEND="bar? ( cat/bar )" and then to mask the USE-flag bar (either globally or for the package cat/foo, only) in the sparc profile. It seems that an analogous masking action in a public overay is not possible, making it necessary to introduce user-inconvenient hacks like REQUIRED_USE="sparc? ( !foo )" and, moreover, if one wants to avoid repoman errors additionally a redundant and misleading RDEPEND="foo? ( !sparc? ( cat/foo ) )"
