The situation with the nvidia driver relies on the part of the API that the Linux devs have claimed is not GPL. When nvidia wants more API, the Linux devs have pushed back against that:
http://linux.slashdot.org/story/12/10/11/1918251/alan-cox-to...
I don't know how this compares to ZFS. But regardless, there's another thing at play here, the method of distribution. Nvidia doesn't ship Linux together with its blob. I think that if nvidia did that, we would have a situation more analogous to ZFS + Ubuntu.
You are referring to symbols marked EXPORT_SYMBOL that the kernel will use when linking non-GPL code as part of the module load. ZFSOnLinux uses only these symbols. The kernel will not make symbols marked EXPORT_SYMBOL_GPL available to it and there are no hacks to access them through the presently GPL licensed compatibility layer. Code that bypasses this when built as a standalone module will not be accepted by the project.
However, there are other, valid examples like Linksys routers shipping with binary drivers for Broadcom NICs.
Besides it should be possible to easily distribute third party binary drivers for Linux and have some reasonable expectation that they work. The fact that you can't is one of the biggest reasons Android updates suck so much.