Have you tried adding device-id = 0x73AB to the SSDT?
I have tried that today, unfortunately to no effect.
I also found out that OC's DeviceProperties will only set values when they don't yet exist, which explains why the device-id was not modified when I attempted to patch it via that method. According to the reference manual, one has to add "device-id" to the "Delete" list as well for it to be overridden.
Well, I tried that, but unfortunately the device-id still remains untouched.
I am perfectly capable of injecting arbitrary properties on that GFX0 object, but the device-id resists for some reason.
Most of what we've tried thus far gets me into the OS, but with the device-id untouched.
The only thing that produces a different outcome is the pure SSDT method of spoofing the device-id.
If I use just the SSDT (no FakePCIID or DeviceProperties) and spoof to either 0x73BF or 0x73BF, it appears to boot up normally, until it hits the black screen issue. It also spins down the fans as if it were on the login screen.
This leads me to believe that an update to WhateverGreen may be able to get me past this black screen issue eventually.
Before anybody asks, yes, I am using WEG with agpmod=pikera. I also tested without WEG on MacPro7,1 and both produce the same result, which is why I think this may be something to be fixed in a future WEG update.
For now I'll run without the new GPU until a macOS or OC/WEG update happens, at which point I'll try again.
Alternatively, if anybody has some other things they want me to try, by all means, I'm up to experiment.