deeice on the #zipit irc channel on freenode.org has ported rockbox as a app onto a busybox/linux embedded system that running on the zipit2 handheld little computer. i would use there work as a starting point. download a deeice zipit image for code unless the rockbox code is available separately. its also a package for the openwrt distro for the zipit that mozzwald maintains. rockbox is not as snappy as it is on dedcated hardware but maybe that old zipit hardware cpu and not being run native? I guess on faster hardware maybe rockbox as a app is faster.