I don't think this rises to the level of awfulness that Mamesick sometimes likes to perpetrate. At least the results are correct and pretty good looking, which I can't say for several Mamesick specials.
That said, this does clearly belong in the core as an optional attribute of screens that are type LCD.
ETA: Regarding tuning, I'm thinking you pass a struct that specifies how many frames to involve in the persistence and how much each contributes to the final image. It could be fiddly to tune, but you need that power.
Also, doing it in the core opens up the eventual possibility of having a pixel shader do it for you, which would be an obvious performance win.
