O plug-in Saft supostamente pode fazer isso (o último download é apenas para o Snow Leopard).
Do artigo barra lateral de safári :
The plug-in goes into the Library>InputMangers folder replacing the previous version. Once launched, Safari then has the ‘Show Sidebar’ command in the Safari menu (this is a temporary position).
[EDITAR]
De acordo com sua observação de que o Saft não funciona em 10.7, aqui está um extrato do artigo Faça o SAFT funcionar com versões mais recentes dos navegadores da Web do Safari :
First, quit out Safari and then open the top-level Library -> InputManagers -> Saft folder. Control-click on Saft.bundle and choose Show Package Contents from the pop-up menu. Within the Contents folder, open info.plist, either in Property List Editor or your favorite text editor. There is an option for SaftSupportedSafariBuild within which you can re-enter the version number from Safari's About Box. Change that, save, and relaunch Safari to enjoy Saft goodness.
[robg adds: Depending on how much Safari has changed, you may find that Saft still won't work, or won't work properly. The Saft author is usually quite good about keeping up with Safari updates, so this hint should only be needed if that ever changes.]