So, to do it, go into the Error Console, and type this line in: Components.classes['@mozilla.org/appshell/window-mediator;1'].getService(Components.interfaces.nsIWindowMediator).getMostRecentWindow("mail:3pane").document.getElementById("tabmail").openTab("contentTab", {contentPage: "https://wave.google.com/wave/?nouacheck"});. Note that Google Wave for some idiotic reason decides that Thunderbird isn't a valid UA to be using, so you have to convince it to disable the UA with the ?nouacheck. I thought browser sniffing died out years ago...
For bonus points, if you restart Thunderbird, the tab will stay open, so all you need to do is login again!