I have a hard time envisioning an L on the current platform, given the current S will probably be redesigned within 3-4 years at most. If we see an L, it will probably be on the next platform.
An L would also mess up the aesthetics of the S. It's one thing to add a few inches in the middle to a 3-box sedan like a 7 or LS but a whole 'nother thing with a swoopy, elegant design like the Model S.