Instead of opening the Google Glass camera app, you could take the picture yourself: http://developer.android.com/training/camera/cameradirect.html
This is even mentioned in the GDK reference:
Building your own logic with the Android Camera API. Follow these guidelines if you are using this method:
- Take a picture on a camera button click and a video on a long click, just like Glass does.
- Indicate to the user whether a picture was taken or a video was recorded.
- Keep the screen on during capture.