Yes they would. You can set him up in two sentences. "He was the Master of Darth Sidious/Emperor Palpatine. Everyone thought he was dead, apparently he's not." "Knowing" who Plagueis is does not require them to read the novel or every single related book/comic. He's quite easy to set up. In fact, the same applies to any character/organization. Almost everything can be set up within a single scene, so that argument which often appears when existing villains are suggested (that the general audience wouldn't know them) really holds no water.