Cover art displays now are possible on Rockbox, in the committed code (i.e., not only by way of a patch).
However, as best I can tell, setting margins for text to appear to the side of cover art is not yet possible in the committed Rockbox code (although there are a couple of patches out there to do this)--my best guess is, that will be up next for committing to the code, if possible, making cover art very practicable.