What a wonderful feature!
Dr Darren says:
Apparently one of the MagicDraw/Cameo customers who suggested this feature was my SysML-obsessed colleague John Brush from 7Sigma. Thanks John for suggesting and No Magic for implementing!
Here's a sneaky trick: Use an outer Package Diagram, turn off display of any text or stereotypes in its frame (but do leave the frame on), set the diagram (frame) pen to white, then export. Voilà! Self managing composite images of multiple diagrams. Absolutely brilliant.