Is there any unwritten rule against hiring already unemployed PhDs?
I don't think there is an unwritten rule, precisely, but I think once you are not employed in academia (even if you have a job somewhere else), your chances of getting a position in academia decrease extremely rapidly.  Competition for positions is so massive, and there are so many well qualified applicants that I think someone who is not currently in an academic or research position is unlikely to be taken seriously.  Not to mention that you typically aren't doing the kind of research and networking you need to get a position if you are unemployed (not always, but often).  I think some fields where it's very hard to get work are a bit more forgiving (though, of course there there is even more competition), but this is part of what keeps people in adjunct positions, since it is a way of staying "in the game."