I have started refactoring my code changes into smaller, digestible patches to use with the mechanism described here, https://wiki.gentoo.org/wiki//etc/portage/patches, but this doesn't work for flexbackup. It starts to work when updating the ebuild with a single line, EAPI=6. If this is not desired, I'm open to other suggestions. There may be patches forthcoming in the future, and I'd like to be able to test them locally without having to modify the ebuild.
flexbackup-1.2.1-r13 now uses EAPI 6
Thanks, Hans! I will file another bug report with my patches for discussion.