You know, if updating is a problem, then just don't upgrade! I don't always upgrade as soon as its out. I upgrade when I have the opportunity to take the downtime. Like I'm two revisions behind on 1 server I have and one revision behind on another.
As long as you aren't hard pressed for a fix or feature that is added, there is no reason to be upset about the downtime.
Frankly, if you are using FreeNAS for production and are that hard pressed for a fix or feature, there is a solution. Buy High Availability from iXsystems. ;)