I guess it would be entirely against T's & C's but it's pretty plain html so have yuo tried value="your chosen keyword" in the <input> tag?
From Googles Terms and Conditions: I'd read that as you cannot pre enter text for the web search boxes in the adsense program. The modification is one, the "must orginate from individual human end users inputting data directly into a search box" Perhaps you should email Google about it?
You aren't allowed to do that. In another forum a guy e-mailed Google and the answer was a definite NO.