The file you uploaded doesn't work, but doesn't matter i better will explain you how to play with it.
Remember the pattern i posted yesterday? It has 3 blocks: Cover, semitransparent dark box, and title of the track. The easiest way for adding Artist text as a fourth block, is copying the title element and modifying. So for the first cover you will have to add
<element type="Field" id="Tracks"
xAnchor="#AlbumCover1.Left" x="0" yAnchor="#AlbumCover1.Bottom" y="-16"
widthDock="#AlbumCover1" width="-2" height="20"
font="Segoe UI" style="Bold" size="9" align="center"
><child fg="TrackAndArtistPanel(86)" id="Artist" ></child>
Modifying the bold text will change vertically its position. Maybe you will only need changing Bottom for Top from the mockup you posted. When you have it in the position you like. Add that fourth box to the other covers too incrementing the values for the different covers
Ps: Thanks for the kind words! The version with the next covers added only had 9 downloads yesterday, strangely not as welcomed as i thought :/