Daisuke Kato as Yasubei?

http://us.imdb.com/name/nm0441961/

[Whoops. I just watched Part 1 again, and Yasubei is the other character in the challenge scene. Kato is playing Fuwa Kazuemon (and I'm sure now that it's him).]

He was Kambei's right-hand man Shichiroji in *Seven Samurai* and Ushi-Tora's brother Inokichi (the "wild pig") in *Yojimbo*, among many other roles.

His first credit on IMDB is in 1949, but his birth year is given as 1910, so he would be old enough.

He appears in the scene where all the retainers gather together to hear Oishi's decision, and appears toward the end in Part 1 to challenge Oishi.

I'm pretty sure it's him, but not sure enough to add it as a credit.