Hi erik,
I've only spoken to Japan on SSB field days as my garden isn't big enough for a decent HF antenna (at the correct height!), which is essential to get over there from the UK. Unfortunately those are of course pretty standard exchanges in the rush for contacts. I have used (by accident) internet linking to the US, when listening in to a repeater which was operating a link & had an american come back. A bit of a surprise. I have considered whether satellite operation would reach over there, but AMSAT contacts are only a few minutes long at best and I've not checked the footprints to see if Japan is reachable that way. Also rather a lot of equipment tinkering is involved and I'm really short on time these days.
A quick look at the Echolink website shows quite a few simplex links in Japan, but no operational repeaters (at the time of writing). Are there any that come up periodically? What times are they active? If so, maybe it's worth registering with echo-link. I'd certainly like to hear about your experiences.
73, Alan
Edit: Found some active repeaters in Tokyo now
Edit2: The UK doesn't have reciprocal licensing with Japan, so I'm not sure whether I can use their repeaters. I guess it counts as 3rd party traffic?