This is a small fix with a big effect: I forgot to initialize the DevicePath-pointer-pointer when registering the ramdisk. I now instead create a mutable reference of a pointer, which is initialized by Rust and automatically converted to a pointer by the FFI. And, lo and behold: The image loads (slowly…), and the RAM disk is mounted.
I also got QEMU to work again, the default behavior did not give the VM enough RAM. It now gets 2G, which should be enough for both the RamDisk and Arch. Just, don’t select copytoram.