scribus

The scribus (open source desktop publishing program) wiki has a page on scaling an image to fill a frame proportionally with a python script. Also on the wiki it is suggested on the DTP vs. word processing page that database driven publishing can be a challenge but they attempt to allow that through the script interface. InDesign may allow javascript to do this kind of scripting, but I haven't tried either program out yet to see how this would actually work. I plan on looking at scribus and alternatives as a means of learning how to generate pdf's with python.