He's got a whole course on "Justice" available for online viewing. Though there doesn't appear to be a lecture on Plato in there, I noted that episode 7 was described by reference to the example Plato uses (referred to on the Plato episode, and more extensively on our Kant morality episode) about whether you lie to someone to prevent an act of brutality (the "Nazis at the door looking for the hidden Jews" example). Here's that lecture in full:
Watch at JusticeHarvard.org.
Get the video podcast from iTunes.
Now, for the most part, this is just a rather labored (i.e. aimed at undergraduates unfamiliar with Kant's difficult-to-understand views) presentation of Kant on morality, but I took a look at this with Plato in mind and found a parallel:
At around 6 minutes in, he describes Kant's view of morality as arising out of our status as non-empirical beings:
"As a subject of experience, I inhabit an intelligible world... to be independent of causes in the sensible world is to be free."
This sounds very much like Plato. To act morally for Plato is to obey the dictates of reason, but this means not merely making a sober assessment of the situation on the ground, but applying a standard that we somehow have innately and can recognize when we turn away from the sensible world.
Nonetheless, Kant's and Plato's ways handling of the case at issue are opposed: in Republic book 1, Plato uses our intuition that of course we should lie to the murderer at the door to rebut Cephalus (the very first one in the dialogue, who is quickly dispensed with and leaves the room). At around 11:30 of the clip, you can hear Sandel defend Kant's hard-line view: Sandel states that there's a difference between a lie and a misleading truth. He doesn't get around to actually stating why he thinks this is until about 21:00: "There is some element of respect for the dignity of the moral law in the careful evasion..., and that is part of the motive." This seems totally unsatisfying to me, but rather than going into my own view of this, I'll let you just watch the video and hear the questioners (they start debating the point around 18:15).
Plato, in countenancing the "noble lie" to keep the guardians in line, shows he is OK with the end justifying the means, or maybe a better way of putting it is that Plato is tolerant of real-world demands on creatures like us in less-than-perfect circumstances, unlike, say, the gods, who would have no need to lie to achieve some end. This looks like an argument against the "one-dimensionality" of Plato's ethics as I described it, but maybe not: the ideal is still singular and simple in a certain way, and the apparent multiplicity or complexity of machinations necessary to reach the good is not a matter of the character of that ideal but of the material world that's in our way of reaching it.
I must say this is probably the most elaborate filming production of a lecture like this I can recall seeing, with multiple, moving cameras, reaction shots (a shot of someone's actual notes!), well-miced questioners, cutting away to video footage (with audience reaction shots of that), etc. I presume there was actual editing, as there's no dead space as different questioners get on the mic or the like. With this level of documentation, the only downside from not actually being there is that you can't actually stand up yourself and ask a question, and to me (passive observer as I am), the advantage of being able to pause/skip/repeat sections more than makes up for this.
If you're not up to sitting still for student-pace video lectures, try these four audio lectures by Sandel on "A New Citizenship."