An app could just do it's own "overlay", by reading the player's display buffer and merging data from that into it's own.

Wouldn't that require the app doing the overlay to be in the foreground? It seems like there are a lot of scenarios where a background app should be able to take over all or a portion of the screen without user interaction. Emphatic being able to jump to the foreground when a song with lyrics tags plays or gpsapp doing a popup to warn about a turn even if you are doing something else for example.

-Mike