As difficult as this advice might be for a prima donna like Bob Rae to accept, the best thing the Liberals can do right now is just sit back, shut up, and allow Jack Layton to implode on his own timeline. The NDP has the impossible job of needing and trying to muzzle dozens of new and idealistic MPs. NDP Amateur Hour will be on full display when Parliament resumes, as it has been in the weeks leading to go time. The best opportunity for the Liberals to gain any significant ground is by attempting to appeal to NDP voters after they've had a chance to peak behind the curtain. Certainly naming Bob Rae as interim leader will not appeal to the center, especially in the most populous province of Ontario.
The downside of an allegedly "centrist" party appointing a very socialist leader is that it becomes extremely difficult to appeal to the Tory left flank. If Bobby makes gains, it will be at the expense of the NDP. Hell, if you add 100% of the Liberal and NDP votes together from the last election, the resulting coalition would have over 180 seats. That has to be goal for the day dream believing coalition supporters. The danger for the Liberals in pursuing a left wing strategy is that they again expose their own right flank. Though I suppose the Tories have absorbed the majority of right leaning Liberals already, otherwise we wouldn't have a majority government.