A Monk of Empty Hands does not take the -4 nonproficiency penalty for improvised weapons.
James Jacobs, Paizo creative director, clarifies this on the Pathfinder RPG forums:
The monk of the empty hand treats improvised weapons as if she were proficient in them, and can wield normal weapons as improvised weapons. This effectively has the same end effect as the Catch Off-Guard feat, but isn't a feat and thus doesn't use up a feat slot since it's a variant class ability. Thus, they do not take a nonproficient penalty when using improvised weapons. The text is not as clear as it could have been, I guess, but hopefully common sense can step in to bolster that if someone doesn't read this post?
(We COULD have simply said, "The monk of the empty hand gains "Catch Off-Guard" as a bonus feat, I guess, but that would have lost the flavor bit about how they often wield normal weapons as improvised weapons.)
Thus, you don't need to take Catch Off-Guard at all, not even for Improvised Weapon Mastery since you don't need to meet requirements for Monk bonus feats.