I think you'd have to rewrite the Rockbox code for displaying album art. I have wished I had the skill to do that - there are a number of manipulations it would be nice to have (e.g. rotating the cover art, either a simple 90 degree rotation in the plane of the screen or using perspective distortion to rotate it around an axis in that plane a bit like how it works in the cover flow plugin). Putting a blur effect would be another thing one could add to a more customisable cover-art display routine. But my eyes glaze over every time I try to make sense of the existing code.
You can just display the cover art as normal, and then write text over it, as some themes already do. But it's not quite the same as making it the background.