If it isn´t possible with a normal HUD, maybe you can build your HUD as a real object in your game, like they do with some helmets or cockpits in other games, and then try to create a post effect, that only affect those objects.
If it isn´t possible with a normal HUD, maybe you can build your HUD as a real object in your game, like they do with some helmets or cockpits in other games, and then try to create a post effect, that only affect those objects.