It seems that the Google Dialer looks at some other properties to determine whether it should allow the Pixel-specific features such as call screening. There are a few Magisk modules that claim to enable it. The newest, and possibly most promising would be Pixelify:
It looks like there are a few things that need to change to get it working. Looks like a JAR needs to be dropped into
/system/etc/framework, then there's a bunch of feature XML stuff that needs to be in
/system/etc/sysconfig, and then a few permissions that need to be set in
I know nothing about building GrapheneOS, or any other AOSP system for that matter. I imagine you could build your own ROM with those files already in place? I also have no idea if there would be another supported way to get those files onto your device at install time before you lock your bootloader back up, but I suspect not because of secure boot.
I'm interested in getting this working as well, but I don't have a ton of time to invest this exact moment.