The Linux kernel currently has code to allow booting an initial root file-system via NFS or CIFS for non-blockdevice file-systems while a new patch aims to allow for mounting of arbitrary non-block device file-systems as root.

Longtime kernel developer Christoph Hellwig sent out this new patch based on earlier work by Red Hat's Vivek Goyal. Extending the CIFS and NFS root file-system support right now in the kernel, this extension allows for arbitrary non-block device file-systems to be used for the root file-system.

One of the motivations for this patch is for easily allowing VirtIO FS root file-systems with options such as "root=myfs rootfstype=virtiofs rw" but can just as well apply to other file-systems too with this patch of just 40 lines of new code.

See this kernel mailing list thread if such functionality is of interest to you.