I've seen a handful of American sets go up for auction on the Japanese sites over the years, so I think there's at least a
little bit of interest. There was recently a Predicta Debutante listed at about ~$300 starting; I wonder if, considering the rapidly inflating price point of Predictas, it will some day become cheaper to import them from Japan than to find them domestically
WRT the service data, the unwillingness to digitize or share is definitely driven by Japan's highly strict IP laws. My aforementioned Japanese video game/computer collector friends go through hell trying to save or digitize
anything. On the vintage computer side, most of the software and disk dumps available online are from American collectors spending sometimes hundreds of dollars on importing original disks to image, fighting the Japanese collectors who also want them to use, or at least "preserve." And after all that time and money, Japan's often-humid climate doesn't make magnetic media recovery very easy at all...
I think the presiding collector mindset over there is basically that if you can't get original copies of service data or (in the case of computers) software, the equipment is to be retired.