FreeBSD supports UnionFS. Does illumos support UnionFS? Does illumos support any overlay file system at all?

Btw, FreeBSD and illumos are having more things in common than Linux. You can learn how NomadBSD is doing things. It has much better performance than OpenIndiana live usb. The most interesting thing is, it supports persistent mode like Linux, too. You can have a fully functional system running completely from the USB.