MacBookPro8,2 late 2011 GPU fix with EFI: how to do it on Catalina macOS 10.15 (dosdude1 patch)?
I'm having the same issue trying to load the AMDRadeonX3000.kext after boot.
I was able to create the force-iGPU-boot script though, by mounting the Catalina system disk as R/W in terminal:
sudo mount -uw /