I actually just thought of a way to add a background behind the soft keys.... !
But before you get too excited, its not a real answer.... but it works... and I don't know why I didn't think of it before....
When using the transparent soft key mod, which removes the black background behind the soft keys (still there is no way to add an image in its place)...... Just add a background to your favorite wallpaper, and it will show up under the soft keys and look just like a background....
The only downside, you have to add this to any wallpaper that you plan on using with a background. But it works, and I think it looks great...
Snapshots:
It is not the most ideal method for adding a background to the softkeys, because it has to be edited into the wallpaper... but it works for now...
If you have experience with any image editor you should be able to take a wallpaper, add a dock or soft-key background to the bottom of it, save the image, set as wallpaper.... and as long as you have the transparent soft key mod installed then your golden....
If you have no experience with image editors, I can take requests.
Im gonna play around with this, see what colors/designs look good. This was just a quick idea.