There's two different things things about the media player on the cover display:
1: Is it possible at all to control what is playing right now? This seems to depend on the kind of notification used by the media player. Podcast Addict, e.g., uses a custom notification by default that shows a progress bar in the notification etc. With this notification, the cover display will show the media player when the Cosmo is closed, but without any podcast title or episode image, and it's not possible to pause it or do anything else. If you configure Podcast Addict to use the standard player notification, though (you may have to kill it from the app settings for the change to take effect), everything works perfectly on the cover display. (Except some glitches now and then when the cover display will only show the media player interface if you open it via the menu, but that's independent of the used apps, I guess.)
2: When swiping from left to right on the media player interface, there is a list of player app icons. The only ones present there for me are Spotify and Google Music. I've never used Google Music on this device, in fact it's deactivated, but the cover display still shows it. Podcast Addict is not shown. I guess that's where the app has to be on Planet's list in order to be shown there.