I'm not sure if this will suffice, but if you hold the Command key while clicking on a link, it will open in a new tab behind the current tab.
You may want to just open a new tab behind the current tab, without clicking a specific link. I have it set to open new tabs showing Top Sites. If I hold the Command key and click the Top Sites "button" on the toolbar, I get a new tab showing Top Sites behind the current tab. For me, this is equivalent to giving the new tab command, except for the tab appearing in front of the current tab (with the new tab command).