I still cannot believe it. After waiting only four months, the Dual G5 XServe has just arrived today. Unfortunately I'm leaving for a two week trip (Linux Networking Summit, Linux Kernel Summit, Ottawa Linux Symposium) tomorrow, so I don't really have any time to play with it.
Just had a quick look under the hood, and it seems like putting some additional drive in place isn't difficult at all. The Mainboard has three SATA connectors (two empty), and if you remove the front panel you can access the two empty 3.5" drive bays. The PCB for Apples hot swap bays is also present, but unfortunately missing the SATA and Drive connectors.
The only remaining issue is getting power to the SATA drive, but that should be pretty easy to find out...
So you might ask yourself why I didn't buy the non-ClusterNode in the first place? Because it's way more expensive and apart from those tiny details exactly the same hardware.
Another interesting part will be bootstrapping Debian/ppc onto that box - without any VGA board and only a serial console. Apparently there is no distribution that really supports installation via Serial console (even on x86)... despite being extremely easy to implement... *sigh*