Since Kindlegen creates the mobi and KF8 parts separately, you could theoretically try to generate the non-KF8 mobi without the metadata and merge it with the KF8 part.
For more information on this highly experimental method see the Generating mobi and KF8 parts of Kindle file from separate sources
However, there's no guarantee that the combined file will be accepted by KDP.
Also there's a distinct possibility that fixed layout mobis will be made available only to Kindle Fire users and Kindle apps with KF8 support. I.e. the non-KF8 mobi part might be discarded anyway.