I am happy to announce that I have managed to get Debian Stretch ARM64 running on the Raspberry Pi 3 with a custom kernel. It required me to recompile the Raspbian kernel with ARM64 support and install the kernel modules, as well as install a few other packages.
I managed to get Debian Stretch running with the XFCE desktop running Chromium and LibreOffice at acceptable speed.
Pi's built-in WiFi currently not working, but Ethernet works fine. I've released a pre-release image you can find at https://github.com/CodeExecution/Debian64Pi under the releases section. This image is in testing stage and you may encounter some issues.
For those who want to build the image themselves, instructions are found at https://github.com/CodeExecution/Debian ... uctions.md
The image is a minimal installation and does not include a desktop environment, but one can be installed by following the instructions on the GitHub page. Once Debian Stretch for Pi 3 becomes stable, I will release versions that have desktop environments pre-installed (such as Debian XFCE and Debian MATE).