Big sigh of relief - managed to front-port the patches to 5.10(-rc4) tonight, which turned out to be even harder than 5.9. 5.10 is designated to be an LTS kernel, so even if future version become too hard to continue to support for GCC LTO, we can ride the LTS wave out on the Rock Pi X for awhile!
Same thing, will test some and wait for 5.10 release before publishing binaries.