This is the perfect place for these questions. It is just a little quiet around here on the weekends.
In short, you want to use the offset feature... but not generically...
Take a look at the offset command and use the button that depicts a drafted/lofted feature. This will allow you to change raise or lower a sketched feature on the quilt (combined surface) of your choice. It is very powerful when you start looking at the options, including projection direction, applied draft, and even radius' on the edges. It is primarily a "stamping" type tool and is perfect for your application. in fact, it will likely take place of all those sketches you extruded to the surface since it uses flat sketches which act on the surface. The key is selecting the right button in the dialog:
Would love to see what you come up with when you are done. Please share the process in a document or blog.