Ideally, the RPI version should be released at the same time as the AMD64 version or shortly after.
I don't know how the RPI release is built, but there is probably a way of automating the build process.
My guess is that up to now, ARM ports of Ubuntu had to use kernel 3.x instead of the version in use on Intel platforms. With kernel 4.8, it may be possible to use the same kernel version on ARM and Intel platforms, which should simplify the build process.
Then, as the RPI3 has an arm64 processor, it should also be possible to build a 64-bit release for this platform.
My guess is that it works in the same way as the amd64 version : all packages are built for the arm64 architecture, but armhf 32-bit packages can also be installed if they're not available as 64-bit (just like i386 packages can be installed on an amd64 Ubuntu).
So basically, a good starting point could be to copy and rename the release build tools for amd64 and replace "amd64" with "arm64" and "i386" with "armhf" in the scripts. The only difference is that there is no installer for ARM platforms, just a disk image with a device-specific boot loader, but this could probably be automated too.
However, these are just assumptions as I don't know how the Ubuntu build environment works. If you have some documentation to point me at, I could start with a bit of self-training.
Regarding Jitsi, it is a Java application with only a few native libraries and as .deb exist for amd64 and i386, building armhf and arm64 versions is not a big deal. The most "complex" part of the job is the integration in the Ubuntu repo.