Needs Interface Builder - parts of the Developer Tools.
• make a copy of Safari
• control click, "Show Package Contents" and navigate to Contents/Resources/YourLanguage.lproj/
• open Browser.nib with Interface Builder
• press shift-command-I to show the inspector palette
• select the home button
• in the inspector select connections from the popup then the target/action tab
• goHome: should be the marked action
• select newTab: as action and click change action button
• save nib file
(you still need a home page defined in preferences for the button to be activated)
(you might want to create a new icon for the button)