I found a pair of Script Xtra that I think will do what I want, i.e. set my status based on whether I'm working from home or not:
http://www.adiumxtras.com/index.php?a=x ... ra_id=4998
http://www.adiumxtras.com/index.php?a=x ... ra_id=6372
What I don't understand is how to turn one of them on.
Do I just put %_isomething into the Custom status message field?