I finished a little Air-app to stream complete jobs to the Grbl and render the result. This is the first “full scale test” running some 7000 lines of sample G-code I scrounged from this thread on CncForum. The Air-app feeds g-code to the microcontroller one line at a time, while the microcontroller generates the steps and feeds them back to be simulated in the Air-app. Everything seems to be working perfectly - the only thing missing now is a test with real steppers on a real machine.
You can get the code for the Air-based test-harness at the github-repository