With virtio-fs I think this is becoming more doable. Of course, it's not quite as fast as running a guest filesystem on virtio-blk yet, but maybe it'll get there.
Isn't that just a binary loader, like you can do with wine/qemu on any Linux? Surely it's mostly just proxying the pipes, replicating the working directory, and maybe keeping some of the environment variables. The filesystem integration seems like the trickiest part of that altogether.