Automate placing custom svg inside template svg for printing a variety of posters

I have to print a number of different posters, each with a template and then a custom graphic within a container. If I can get the id of the container within inkscape, how can I script cycling through all 100 graphics in a folder to add them to that container?

Note that while I'm using Ubuntu, my solution should ideally be cross-platform either using a batch feature of a cross-platform software or Python (with svgwrite package).