Let me repeat: I was wondering if anyone on this site who has personal expertise or training in Sumo would choose to comment on Sumo's availability outside of Japan, applicability to Aikido, similarities and differences between the arts, etc.
Is the overlap in practitioners limited to Japan in the past (and possibly present) or is there lots of Sumo training ongoing outside of Japan as well (in the US, for example)?
There is a good deal of amateur sumo training available in the world. Not a lot, but it's not limited to just Japan. Here's a link
Explore that site, and see what you can find.
FWIW, Kosei Inoue, probably the best competitive judoka in Japan right now, has begun making occassional training trips to a professional sumo stable in preparation for the Beijing Olympics.