I got the following rumor from a new source:
Yes, Panasonic is developing a Super35 L-mount variant of the AU-EVA1 5.6k pro videocamera and a complementary Super35 S5 SLR body type. Panasonic is working to offer L-mount across its professional video lines as a long-term option to EF-mount. They are doing so in both FF and Super35 formats, and in both videocamera and SLR S5 body styles. They currently have 4 prototypes in the field with one studio in North America doing field work. Expect a 2021 release. We have seen and heard nothing about further developments for 43 systems.