I'm a US citizen living in Tokyo, so might be able to help if people have questions. I am not nearly badass enough to consider myself mustachian, but I do appreciate the outlook.
For investing, I had a Charles Schwab account that I kept open when I moved here, and I continue to use it. Schwab has a local account at Citibank Japan, so it is quite easy to transfer funds from my Citi account to Schwab's account and have it automatically deposited in my Schwab investment account. It's also a lot faster (usually overnight) and cheaper (300y versus 3000-4000yen for an international wire transfer). The only downside is that I have to visit a Citibank branch to put in the form.
I don't do any investing via Japanese banks or brokerages, though I have looked at it a couple of times. In the end, I've always decided it's a lot simpler just to stick with Schwab. If you are going to look at opening an account with a Japanese brokerage and you're a US citizen, be sure to ask if they have restrictions on the types of investment products you can buy. Many will not let US citizens buy mutual funds based on US companies and products. If you're hoping to do low-fee index fund investing a la the Bogleheads, this can make things very difficult.