Okay, so, that didn't work. But at least I now know why.
On the third page of Vision Brewing's "Basic Recipe" (doburoku), under "When to Halt the Brew" one would find this sentence:
"It is often a good idea to stop the process after about 8-10 days when the brew is tasting at it's [sic] best i.e. Sweet, slightly bitter and acidic. When the brew gets to the end of the process after two weeks it is often overly acidic and not so tasty."
The mistake I made was that, after a week, I thought the fermentation had stalled and needed more time. So I left it to sit for over four weeks. Sure enough, my brew was "not so tasty" (I ended up dumping the entire batch). If I had stopped it after one week it probably would have been considerably better.
I'll be giving it another shot when the weather turns colder (which is, naturally, the best time to brew doburoku, as the slower fermentation should increase the quality of the brew). I had no trouble with any of the rest of the process: my koji-kome turned out great; the rice steamed up nice and dry; the fermentation process worked just like it was supposed to. I have every confidence I can master this simple process that rural Japanese have been performing for centuries.