I made some progress on a full-touch
cabbiev2 for RaaA (Rockbox as an Application) a while back, but the highest resolution port I made is 480x800 (portrait).
I essentially stopped working on this, as not only was the style and functionality a contentious issue with a few very vocal yet influential people, but the direction in which RaaA was headed was very much up in the air, and still is. I didn't want to invest more time and effort continuing the ports until everyone had a clear, unified vision.
And now, I have bigger things in mind for the default theme. A "secret project" if you will.
But, I digress...
If I parse your post correctly, it seems to me that you are simply wanting (static) reference images? If this is the case, then I would suggest that you simply capture some screenshots of Rockbox in different states and resize them in <insert_editor_here> as opposed to painstaking mocking up images by hand. Rockbox can easily be configured to capture screen dumps by setting the toggle:
System -> Debug (Keep Out!) -> Screendump
(upon selection of this item you will see a splash on-screen showing the state of the setting)
After you have enabled screendump, USB insertion triggers a screenshot, and the images are saved to the root of the device. If you do not actually own a Rockbox capable device yourself, you can also do this with the simulators.
A suggestion I will make to aid you in getting uniform images would be to disable scrolling in the display settings, as scrolling text can look all different kinds of weird when trying to get a screenshot.
[Saint]