I’d say your idea for using a blueprint is the best solution. The print dots are likely fading in from something like motion blur or adaptive exposure, which means it’ll be different on different configurations, so it’s best to make it a deliberate process. I haven’t tried hooking parameters into a blueprint yet, might have a crack at it today and see how it’s done.
One thing about print dots is they obviously only draw in the darker areas of the final tonemap (by using it as a mask). This means any slow changes in lighting will cause them to look like they’re creeping in, so that’s another mechanism for making them appear.