You have to compile it yourself, as it is now. Which is a pain, as ARM32 QEMU on AMD64 has some issues and, in practice, you need to setup 32-bix x86 virtual machine and compile ARM32 flatpak there. I wrote the script to package it, but it is not pushed to Flathub. Flathub build servers are running on ARM and would be able compile all without issues. However, for publishing at Flathub, some extras are probably needed. I don't plan to work on it, but you are welcome to contribute. Corresponding issue: https://github.com/mirukana/mirage/issues/1