When you run extract2g (-4 -A) against the iPod firmware.MSE file, you end up with nine partitions. Eight of those are encrypted, so not much use right now (asides disk.fw and osos.fw, which you can force-boot using iRecovery). The ninth partition is the resource partition, which is not encrypted, so you're able to extract the contents (a FAT16 disk image, no less).
Use the following command to strip the header and extract the file:
dd if=rsrc.fw of=rsrc.img iseek=2 count=284672
(If you're not working from the 1.0/36A00403 firmware bundle, you can strip the first 1024 bytes from the rsrc.fw file, and run fdisk on it to find the sector count - 'fdisk -e rsrc.fw'. For the 1.0 bundle, the sector count is 284672).
Double-click the resulting .img file on Mac OS X to have it mount so you can browse it. There's not a lot of interesting stuff there (fonts, etc), but there are two files of interest: SilverDB and SilverImagesDB. SilverImagesDB contains all the image resources for the OS; wallpapers, icons, buttons, etc, including some curious leftover pieces like camera shutter images.
'Silver' I presume is the name or codename of the UI framework on the iPod's Pixo OS (similar to how 'Purple' was used to describe iPhone apps).
Maybe once we find a way to flash a modified rsrc partition back to the iPod (remember, it's not encrypted) then you should, in theory, be able to modify wallpapers and other images/resources.