Not surprising that they never heard of Sirius. It might only be US
satellite coverage anyway...if I were you I’d contact their US customer
service and pose these specific questions. Other, non-satellite services
like Slingbox, which is Internet-based, are known to work in Japan with a
US account, but satellite stuff is more tightly controlled and regulated;
once you leave their legal contracted and permitted territory, it’s rather
unlikely IMO that a US satellite service provider will function for you
(especially in Japan). Good luck!