Put binfmts in /usr/lib/binfmt.d
It turns out update-binfmts is a debian exclusive piece of infrastructure. For better cross-distro support, install the binfmts to /usr/lib/binfmt.d, which is handled by systemd.

Problems: the syntax is very different, systemd binfmt isn't as easy to configure as with update-binfmts, might be conflicts if installed in both places and running debian