I just built the zfs 0.7.9 packages from debian sid and uploaded to omv-extras 4.x testing repos. These are working on my test VM with the 4.16 kernel. I didn't test against the 4.14 or 4.15 kernel though.
That's kind of a relief. If 4.16 will work (eventually), great.
I've built OMV4 a few times on real hardware, installed the kernel headers, didn't install the 4.16 update, pinned the 4.14 kernel in OMV-Extras, and the ZFS plugin insisted on building a module for the 4.16 kernel, regardless. All attempts ended in an error like @hoppel118's above.