By editing your ~/Library/KeyBindings/DefaultKeyBinding.dict file you can set up a gigantic subset of emacs navigation keys (including almost-but-not-quite versions of C-k and C-y) in every Cocoa text widget on your computer, including Safari textareas.

Do a search for DefaultKeyBinding.dict, or just start with mine:

http://dl.dropbox.com/u/5431919/DefaultKeyBinding.dict