I think you're being way too generous in saying that Robert Downey Jr. even showed us as much as nominal amount of "squat" in *Chaplin*. For me, Charlie Chaplin is GOD, and I found that movie somewhat disappointing. If it wasn't for Downey's mild resemblance to Chaplin, and some damn good directing from Richard Attenborough, that movie would've been more than somewhat disappointing.
Perhaps this is a crazy choice, but I would have cast Bronson Pinchot in the role of Chaplin. I believe Pinchot is capable of doing a role like that (if given the opportunity), and he could capture Chaplin's physical humor perfectly.
Loved your suggestion of Downey following McVeigh's act in Terre Haute. :)