Yup. It's called FUSE and ZFS is available that way as well. The problem with running file systems in the user space is that they're generally really slow since

Yup. It's called FUSE and ZFS is available that way as well. The problem with running file systems in the user space is that they're generally really slow since you're chucking lots more data backwards and forwards between the user space and kernel space. So kernel drivers are much preferred in this instance.