We met at Photokina! It may have been me that suggested this to you. We are mapping out bad channels on X900 and X890 printers by customizing our K7 and K6 curves. But - you can not recover the X900 print head with a bad Cyan channel to print with color.
For your 9900 with a bad Cyan channel - that is normally where we put shade 2. We would map shade 2 to the orange or green channel instead and just null out the Cyan channel.
A lot of Piezography options remain for these printers when you have as few as 5 or 6 working channels.
regards,
Jon Cone
Piezography