So I’ve got my pi 3 running in 64 bit mode with Gentoo. I’m slowly updating ROS ebuilds to work on arm64, but have not done them all yet or tested ROS compilation on the architecture (because I don’t want to mess up my portage tree, as getting 64 bit mode was quite a hassle).
That said, I might attempt a 64 bit ROS Gentoo image, and could upload it for everyone if there is demand for it.