The latest upstream release of dev-libs/OpenNI2 does not include support for arm64. Neither does the upstream repository at https://github.com/occipital/openni2.
However, patches to add support for arm64 have been available for a while. Other distributions have since created their own releases of OpenNI2 that appear to include a multitude of patches that have been submitted, but never accepted, to the upstream repository.
Simply by modifying the -9999 ebuild and switching the source to the "aarch64-and-ppc-develop" branch of https://github.com/zeldin/openni2, I was able to build the library on arm64 and use it to build and run OpenSCAD.
It would be great if the patches could either be included in the ebuild, or the -9999 ebuild be switched over to a fork that is not quite so abandoned.
Using the HEAD commit from the "aarch64-and-ppc-develop" branch of https://github.com/zeldin/openni2, removing one hunk and and turning it into a separate patch as the codebase has been refactored, allows dev-libs/OpenNI2-2.2_beta2-r1 to build on arm64.
It may build on PPC too as the patches address both. I can't test that.
The patches are in my devspace at https://dev.gentoo.org/~neddyseagoon/OpenNI2/
I can confirm that adding Roy's patches to the 2.2_beta2 ebuild enables me to build the package on ARM64.