That's not allowed by the Google T&Cs unfortunately... though, I wonder if I can do it with a little Javascript (you're not allowed to modify the Google generated code at all, but it might not disallow Javascript tweaking of form values).
Ah-ha - it seems there is a way to get the Google code generator to do this - you select the sub domain in the sample output and it makes it the default option in the generated code. If you subsequently press the update code button it will reset it though. Oh, and Google don't seem to have documented this!
So, the search is there and all without hacking any Google code, without any Javascript, and without breaking any T&Cs :-)
Regards, Dave.