Both these ideas have come up before. Yet nobody seems in line to do the work necessary to get such things in an acceptable state. Remember, Rockbox is a volunteer project.
I
would be interested in doing this if I had some understanding of how Rockbox renders stuff compared to X renderers on Linux for PCs. Theres awesome anti-aliasing in later versions of XFree86 and X.org, but something tells me that Rockbox is using something else.
As far as visualization goes, there are some nice 2D opensource examples in xmms (the code will likely be in GTK, but good enough to get some understanding).
Then comes the problem of processing speed... I would think that Gigabeat has enough power to do all those things, but its the other targets that I would be worried about. Would Rockbox include/develop/support a target specific feature?
Considering how much time I spend using Rockbox and my general interest in it, I would definitely consider adding to it.