Sorry to rain on your parade, Sunpower, but 4-5 teachers that you know is not much of a sample. Of course, p/t work is certainly available for people with BAs. No one would deny that, however, p/t work is not very good. I would rather have f/t work in a university (which I do) than 4 p/t jobs. It is the benefits, which only f/t work can give you, that makes this job worthwhile. Eventually, p/t work may lead to f/t work which is the only reason why part-timers stay on.
There will always be exceptions, but don't give people the impresson that universities are filled with people with BAs because they aren't. If you already have some kind of connection with a university than your chances of getting a job go up significantly. If you are applying "cold", do not have a Masters and are unpublished, I would say forget about f/t university work in Japan.