The fbdev driver is the only one I've used that isn't on EAPI 7, so it causes building an X server on the host system when installing it in a sysroot. It looks like the only difference when changing to EAPI=7 and xorg-3.eclass is that the README and ChangeLog aren't automatically packaged.
Probably it is the best to prepare a PR with your well tested ebuild and link here with.
Yes, like with the other bugs you've filed... I accept patches.