So not sure what you're trying to say here.
that flashing custom bios is possible
NOTHING CONCLUSIVE--
I tried reading the several threads that I found after searching earlier posts in this thread concerning user-modded 5700 series AMD graphics cards. The user claiming to be attempting BIOS mods and uploading them with ATIflash has poor English or is deliberately using LOLcat-speak. Nothing posted is legible enough to make sense.
Other long-time gurus explained that that the latest AMD cards have encrypted and signed BIOS. The Navi series won't accept a BIOS flash the way Polaris and earlier cards will.
If you can point me to a legible post or review explaining how to alter a Navi BIOS and flash it, please do. --scryptr