Maybe instead of patching the kernel the "init" process of the VM could set up a seccomp-notify sandbox to handle the socket syscalls in userspace to back them

Maybe instead of patching the kernel the "init" process of the VM could set up a seccomp-notify sandbox to handle the socket syscalls in userspace to back them the tcp/udp sockets by a vsock (I think that read/write or sendmsg/recvmsg would work without userspace handling because they get the vsock fd).