Does the 9111-285 have a headphone jack? Can't really be a "workstation" without a headphone jack can it?
With regard to more drives, I don't know how the backside bays connect to the bus, ribbon cable or jacks or whatever, I assume the DVD ROM/DVD RAM/CD players that are supposed to plug in back there are SCSI, couldn't you just shoehorn a SCSI hard disk back there and see if the system can see it? Also, I'm not sure what the bug is that prevents X from talking to the GXT4500P or GXT6500P but it's got to be fixable, same with the sound card, there are a ton of open source audio mixers there's got to be at least one that can be hacked to interface with the audio.
Also, do you have the AIX version of Catia? I've got the IRIX version on CD but haven't tried installing it yet because I'm pretty sure the installation CD doesn't come fully equipped with a functional license key generator....