If you want ZFS, skip sysinstall entirely.
Either use the bsdinstall-based CDs, or the PC-BSD CD. Those both support ZFS installs (and gmirror installs, and anything you can do with [man=8]gpart[/man]).
The bsdinstall CDs are really nice, as the installer is a LiveCD with several opportunities to "do it yourself". Drop to a shell, use gpart to configure the disk, create the filesystems, mount them to wherever it needs them (it tells you), then restart the installer and away it goes.
The disk partitioner is also much nicer in bsdinstall.
You can get the bsdinstall CDs
here.