When leaving Japan, I guess I should relinquish my Permanent Resident
visa, but I'm curious: what happens if I ever need to come back long
term? Do I need to start from a work visa sponsored by an employer
again? Or is some written promise of employment good enough to get a
spouse visa while out of the country?

It's sure a damn waste to give up a Permanent Resident visa,
considering it took nine years to get it, but I won't be able to fudge
how long I've been out of the country if I need to return again.