First, I would like to thank the archlinux community.
To get an audiophile Dockstar running. I had to use two Dockstars: one as a NAS and tftp server, the other as a mpd player booting and running on tftp. You might see here.
The usb transport -> spdif is the April Stello U3 reviewed here.
The configuration on my u-boot (jeff) to be able to boot via tftp is here.