Looking back at the old chapters, I don't think either of Danzou's bodyguards are Sai's sempai:

http://www.onemanga.com/Naruto/285/09/

However, I also renewed my suspicions of Yamato after rereading this page:

http://www.onemanga.com/Naruto/286/17/

Apparently the anime made it out to look like Yamato was just going through Sai's stuff, but there's no reason why Sai should have his sempai's mask...