[edit]Just noticed this was for the search single press, not long press. That's a different story, sorry.[/edit]
In order for an app to appear when you long press the search button it must have an intent filter like:
Code:
<intent-filter>
<action android:name="android.intent.action.SEARCH_LONG_PRESS" />
<category android:name="android.intent.category.DEFAULT" />
</intent-filter>
in the AndroidManifest.xml
If you're interested in doing it yourself then I imagine if you just modding the Gesture search application and added this it would work. Alternatively it would be straight forward to write a program that has that intent filter and launches the program of choice (either hardcoded or cooler would be to have the user able to choose the application the first time it's used).
If you want however I can quickly throw together an app hard coded for Gesture Search and post the APK and the source.