I don't think I can, I'm using gpl code from other parts of the kernel. I'm not sure I would want to either, I put a lot of work into this and the gpl gives me more of a feeling of ownership.

That said, there's nothing stopping you or anyone else from reworking my code into a (gpl-licensed) FUSE driver. I don't think it's a straightforward task, but it can definitely be done.

I don't think it's a licencing issue. It's implementation as both kernels uses different syscalls and have different architecture.

Syscalls are mostly the same, but indeed, the interface between the kernel and the file systems is very different. However, code which implements that interface on the file system is a relatively small part of the whole thing; most of the code should be reusable.

Historical note: FreeBSD used to support XFS; I believe it was ported from Linux.

It takes a lot of work to get the Linux GPU drivers to build for other operating systems.

True. Although it’s way easier than it used to be, thanks to linuxkpi layer - the piece of FreeBSD kernel which implements various Linux kernel APIs.