Homebrew PCBs

>The CD tray has some markings on it with White paint and ofcourse
>the tabs on the front edge.  I am guessing that either the tabs or
>the markings are somehow sensed by the printer.

I've heard that a real CD is sensed there, including borders and the 
center hole.
Try to print something like PCB drawing on a genuine CD first,  then 
replace it with a PCB disk without the center hole.
Cristian