The Internets failed me -- apparently no one has seen fit to write up anything in English about mask-wearing etiquette in Japan.
Luckily, I have colleagues who have worked in Japan, so I can tell you (and the Internets, hellooo Internets) that it is acceptable to go out to dinner with your coworkers sans mask, even if you are totally losing your voice. Which is good, because eating with chopsticks is hard enough, never mind cutting a teeny little hole in the mask through which... no I'm joking. No one eats with a mask on, not even lepers.
Anyhow, dinner was nice. I'm testing the theory that enough sake, shoju, and plum wine will cure laryngitis. Chestnut shoju and plum wine? Very tasty. (The sake, we already knew was tasty, but you might have been wondering about the others.)
We'll talk about my voice later.
Okay, I did find one site, but mostly it asked the question "am I a dork if I wear a hygenic mask and I am a big white gaijin?". I think, if you are asking this question on an Internet newsgroup, the answer is probably yes regardless of the attitude of the Japanese towards foreigners who adopt their germ self-containment practices.