> Would it be possible (read: legal) for someone to modify the ZFS source to be more easily buildable into the Linux kernel, but then distribute it as source code only
Yes and that has been done. The ZFS on Linux[0] distributes a DKMS source package that each user of ZoL uses to build a binary kernel module. DKMS automatically builds the module without manual interaction from the user, similarly to installing a regular binary package.
This is legal but the resulting kernel/zfs module is non-redistributable.
Indeed, it's possible and even mostly-supported (at least on Gentoo) to compile the code into the kernel; i.e., not as a module. I have a zfs system without kmod support (reduces a huge attack surface and just seems cleaner).
Yes and that has been done. The ZFS on Linux[0] distributes a DKMS source package that each user of ZoL uses to build a binary kernel module. DKMS automatically builds the module without manual interaction from the user, similarly to installing a regular binary package.
This is legal but the resulting kernel/zfs module is non-redistributable.
[0] http://zfsonlinux.org/