The schematic shape is built through code - each port is added into the shape using code. So changing this shape will be tricky and you have to be careful. In other shapes it is easier to add custom properties as you need.
I never did get an answer or solution. I currently am manually entering what we use for the system device name and location in text boxes. Not very happy about it. Dtools seems to be very limiting to how someone thought a system should be documented and one thats all in one room and on one drawing.
In the upcoming service pack you will be able to display custom properties, location, zone etc in any of the D-Tools shapes by customizing the shapes. An article will be posted on this soon in the support wikiblog.
However the schematic shape is built via an object model to add schematic ports dynamically and it will not be possible with only this shape even in the service pack.
Adding Custom Property to Schematic Stencil
Linear Mode
