Ask HN: Is lifting Linux FS drivers to user-space possible?

The current state of linux FS drivers for Mac is not good. I was wondering how difficult it would be to lift the exististing file system drivers from kernel space to user space. Would one of the following approaches be feasible?

• write a wrapper, which would provide the kernel API ? • reuse the code as much as possible by stripping it's dependency to the kernel ?

What are the possible problems? I suppose that if it was straight-forward, it would have been already done.