Um Monge de Mãos Vazias não toma a penalidade de não-proficiência por armas improvisadas.
James Jacobs, diretor de criação da Paizo, esclarece isso no Fóruns de RPG de Pathfinder :
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.)
Assim, você não precisa pegar o Catch Off-Guard, nem mesmo para o Maestria de Armas Improvisadas, já que você não precisa satisfazer os requisitos para os talentos de bônus do Monk.