I'm currently in the process of adding a google search box on one of my site. I see that with the current <input> text field the value is blank. Can we set/add some text on the textfield?
Google doesn't like you messing with the code once you copy it from your Adsense members area. My guess would be that if you cannot input that text when you are setting up the parameters for that search box in the Adsense members area, they won't allow it. I wouldn't risk it. But as always ... when it comes to Adsense TOS, don't even bother asking anybody else but Adsense. They have it written out, plus you can contact their support department.
Against TOS. Even if you're not making any unethical changes you're gonna get caught sooner or later. Not worth it.
I suggest to send an email to adsense team and ask them about this. Maybe you'll post the answer here.