Any more documenatation of your experience controlling steppers on an XY grid through Processing?
When I switched to larger stepper motors, some smoke escaped from the motor controller board. I haven't been able to get it all back in, yet.
That code would be a good starting point, then. Create the sketch in Processing, and a button. When the button is clicked, send the sketch points, one line at a time, to the Arduino.
I'm not sure how you would be able to erase a mistake, though. I've used other languages to try to write a decent sketching application. The relationship between a bunch of pixels lit up on the screen and a known element (line, circle, polygon, etc.) is the hardest relationship to define. Selecting an object means knowing what pixels go with what object. Particularly difficult when two objects overlap. Do you pick the one on top? To do that, you need to know the order that the elements are drawn.
Scaling, panning, and rotating (if you want to support 3D) complicate matters considerably.
OpenGL handles many of the drawing issues (panning, scaling, rotating), and the what-pixel-goes-with-what-element selection issues. It has a rather steep learning curve, but is integrated with many higher level application development environments, like Windows forms.