So, in the sentence 彼が持っているのは二百円です, I interpreted は as marking a subject, and in particular, marking known information (彼が持っているの). This causes the focus to fall on the right half, as described above. I tried to express this generally as "the pattern AはB emphasizing B, not A". Does this make sense?
Yes, basically I agree... at least this is what I learned in 言語学概論(Basic Linguistics) class at college I think. I don't know if this would be any help but I remember learning something like this when I studied 情報構造(information structure), 旧情報・新情報(old/new information), 焦点(focus) etc.
私は山田です。---
When you're asked your name, you'll probably say 私は山田です, not 私が山田です, and you'll put the stress on 山田です. 私は part is the known/old information and 山田です is the new information and more important, so the focus is placed on 山田です.
私が山田です。---
When someone who doesn't know you is looking for you, asking 山田さんはどの人ですか?/誰が山田さん?, then you'll probably say 私が山田です, not 私は山田です, with a stress on 私が. You'll probably say the 私が with a slightly higher tone than 山田です. This time 山田 is the old information and 私 is the new information, and the focus falls on 私が, not on 山田です.
There might be some cases where the focus falls on A in the pattern "AはB":
りんごを食べたのは彼です。---
When you say りんごを食べたのは彼です (or 彼がりんごを食べました。) as a response to 誰がりんごを食べたの?/りんごを食べたのは誰?, you'll probably place a stress on 彼です, because 彼 is the new, more important information.
りんごを食べたのは彼です。---
However, when you want to say "He ate the apple, but (it's not his fault, because) I made him eat it...", you might say りんごを食べたのは彼です。でも、食べさせたのは私です. Here, you'll place the stress on りんごを食べたのは, not on 彼です.