Well, thought I'd give it a go - step off the cliff so to speak. Macintosh HD being one of two HFS+ Journaled partitions on an internal disk. Result being that it failed to complete the initialisation:
Unmounting disk
Erasing
argv[0]: /Library/Filesystems/zfs.fs/Contents/Resources/zfs.util
argv[1]: -v
argv[2]: Untitled
argv[3]: /dev/rdisk0s2
zfs.util called with option v
Mounting disk
Could not mount disk0s2 after erase
File system formatter failed.
Operation failed…
sudo zpool import -a doesn't show it up as a pool, and it still appears in Disk Utility as an Untitled partition of the disk. Running the erase on that a second time creates an unnamed partition described as an Uninitialised ZFS Dataset. All of this appears to have no negative consequences on the other partition on the disk. Diskutil list shows up as follows:
192-168-1-2:~ sadmin$ diskutil list
/dev/disk0 (internal, physical):
#: TYPE NAME SIZE IDENTIFIER
0: GUID_partition_scheme *500.1 GB disk0
1: EFI EFI 209.7 MB disk0s1
2: ZFS 248.1 GB disk0s2
3: Apple_HFS SierraServerClone 250.5 GB disk0s4
4: Apple_Boot Recovery HD 650.0 MB disk0s5
I'll fool around further and see what happens.