What you're asking for doesn't exist yet. You can overlay an app on top of the Empeg's display, and as long as you don't bind any buttons, you can use player functions normally while that app is running in part of the display -- but you need to select it from the menu. You can't just have an app open up and start using part of the display.
This is one of several improvements to the userland interface I've asked for from Mark, but then Hijack started getting a lot more focused on more consumer-facing things like bootup-animations, IR translations, the web interface, etc. Now that those things are stabilized and world-renowned, I'm hoping that Mark's next foray into Hijack development focuses on what we 3rd party app developers can use to make our apps more useful.
I have about 3 or 4 things in mind for a user app when/if this type of thing gets implemented. With the stuff that's in /proc/empeg_notify, we could actually create "skinnable" info modes with such an animal. Just have said application startup with the player, and it would run "in front of" the player's display. You don't have everything you'd need for skinning the menus and all (the app wouldn't have a good idea of what menu options were selected, for instance, except maybe via screen scraping) but it'd be pretty easy to replace the line info mode with a customizable one that displays album titles, track numbers, comments, whatever.
That, and my lyrics scroller, are two things I have thought of that would benefit from being able to run without being fired from the hijack menu.
So whaddya say, Mark?