KVM Backend for VirtualBox. With our current development model, we cannot easily accept pull requests here. If you'd like to contribute, feel free to reach out to us, we are happy to find a solution.
From what I understand on the website, that should be possible. However, we currently don't focus on binary packaging so we could only assist when something doesn't work.
Since virtualbox-kvm doesn't use out of tree kernel module can it be packaged as flatpak?