Kernel patches have been in the SA1100 linux tree for a while - 2.2.9 ones will be there soon.
Not all of the code is GPL: the player code most definitely isn't, as some of it is licenced. The source that *will* be released is for things like the transfer protocols, and the UI. The UI is currently *not* in python - we had to back out of this in the first release due to lack of time: we were just pouring weeks down the tubes due to problems with Python in a multi-threaded environment - so that in the first instance, UI mods will most likely be done as dynamically linked C++.
The unit *is* open: you can pump the hard disk with either of two disksets (and swap when you like) - player (locked-down player, as supplied) or developer (shell, zmodem, gzip, tar, this sort of thing). With a host linux box you can cross-compile code for the unit and then play with it.
Remember we're a small company, and software documentation takes time, as does getting code ready for release (espeically when it's source). It'll come!
Hugo