I've ordered the Beastgrip 1.55x MK2 lens, and I also found an adapter that I can mount my Neewer 1.55x onto, so that will mean I can test it properly on various lenses.
I have no idea which lenses these will work on, but combined with the 2x digital zoom of the GX85 or the continuous digital zoom of the GH7 any coverage issues should be able to be overcome.
You'd think that cropping would make the FOV too narrow, but actually it's perfect because with a 1.55x stretch factor it makes lenses far wider, so cropping narrows them back up again, and cropping also lowers the resolution of the final image, but that's fine because we're interested in a flawed and character rendering, not clean soul-less images, so it's potentially an advantage.
Given that coverage might be an issue, this might really suit the GoPro ILS as well.