After my first post, I've been doing some more digging (I haven't looked into all the particulars myself in quite some time), and I came across
this page. Which states:
If you are going to go to Japan on a tourist visa, it used to be that you had to leave the country in order to change your landing permit to a working visa. Most chose Korea because of shear proximity and cheap air fare. This has now changed and this requirement is now being waved by officials at immigration.
There's actually a ton of info on that site. I'm not sure how "good" any of it is as I've yet to look at any other sites, but it's certainly encouraging.