Japan involvement in a Champions League style format was reported earlier this year;
https://www.smh.com.au/sport/rugby-...n-to-keep-japan-involved-20200514-p54sz1.html
. so whilst, yes Japan (JRU) has been looking at doing their own thing I don't think no involvement with AU/NZ n' co has been off the table. Australia is also a more attractive market for Japanese companies than NZ so their might be some appetite for them to see this as an opportunity to be involved and create some extra value out of their existing investments.
I'm not sure about the politics but I got the impression the new League was very much about the JRU trying to untangle itself from the Corporations which currently run the Top League so I'm not sure what the dynamics and different views are. Perhaps someone closer to Japanese Rugby might have some insight.
NTT (Nippon Telegraph & Telephone Corporation) has made some large property purchases recently, perhaps they'd like to purchase an interest in a AU Rugby competition.