There really is no need to give every user a single slice. I would even recommend against that because it only creates unneeded administrative overhead. Just create one large slice for /home and let your users run amok there. If you feel some users need to be limited in their filesystem usage then look into applying quotas.
I have to admit that I only skimmed the thread, but I'm still a little puzzled why you'd want your home directories to be put on UFS instead of inside a ZFS pool. Although those quotas I mentioned above work perfectly to help protect (and/or divide) the available space it is by no means comparable to the diversity and flexibility which ZFS can offer.
Not to mention filesystem safety. Assuming your ZFS pool resides on multiple disks then your data is safer within ZFS than on a single UFS slice, something which I think is very important for user data..
(edit): disregard that last comment, I confused this thread for another and forgot it were both 1 disk situations. As such the last comment doesn't really apply.
I did it this way because I had a second disk to use with freebsd. I found out how to do it with ufs. zfs is more complicated for me. It was hard to find how work with it and get the /usr/home directory from zfs to /home on ufs.
honestly that was the only way I felt comfortable doing it. I also though about that, but was not capable to find out how to do it.