Safari's scripting dictionary is truly, well, pathetic; especially in comparison to, say, OmniWeb. Safari needs a robust scripting architecture, including support for tabs. Heck, I'd be happy to help work on if it someone could point me to the right chunk of code... Hote: this request mirrors <rdar://problem/3226624> cf. http://daringfireball.net/2003/05/safaris_unscriptable_tabs.html
This has nothing to do with WebKit. Safari bugs should be filed at bugreport.apple.com.