I'll be playing around with DragonflyBSD Hammer2 (and multiple offsite backups) for a home NAS over the next few weeks. I'll probably do a presentation about the experience at the Montreal BSD user group afterwards. It does not require as many ressources as ZFS or BTRFS, but offers many similar features.
As for OpenBSD as a home NAS, I'm sure it would be fine if you're diligent about backups. You might also want to get a small UPS for it (I bought a refurbished APC for 80$), so frequent but short power interruptions do not require an FFS integrity check. (I do this for my router)