Unsure how this would bargain personally... because if you own an app long enough, they would likely see all of the wallpapers after enough time. I think the best way to prevent fingerprinting is to be homogenous with the userbase rather than constantly unique, with everyone using the same, default background.
To my very feint knowledge that's also why GOS doesn't have a set list of them. Plus, all black is energy efficient anyway...
Maybe another way is to have apps all see the same wallpaper when its actually different when displayed in the launcher? Although with screen recording apps and overlays this would fail to do it's job... I think this would be too difficult to find a safe and working implementation for it plus it could take too much time to develop something to counter something so small. GrapheneOS development do say already they have issues with getting desired features due to limitations with developers and whatnot