After some fruitless attempts to extend the usable canvas of DCE ChemPad with some kind of self-made view port, in order to allow drawing of larger-than-screen structures, I think I now found a more painless way.
Simply wrapping the DCE Control in a HorizontalScrollView was not enough, because the ScrollView intercepts too much MotionEvents, but after subclassing HorizontalScrollView and overriding onInterceptTouchEvent(), it seems to work as expected. The code needs some more polishing though to call it working though.
As you can see on the screenshot, the structure now can be scrolled left and right as needed to draw more than would fit on the screen. The thin line on the bottom separates the drawing from the scrolling sensitive area, because otherwise the whole screen would be scroll sensitive and drawing while scrolling is not very precise. The screenshot is from a NexusOne with Android 2.2.