I would love to have the possibility to use a 2D drawing as an underlay, both in designviews, and also in plots. When a performance is up and running, most of the paperwork is adapting plots to different venues, as the performance goes on tour.
The plots could probably be solved easily by just letting the insets have transparent background, and then use "Send to back/front".
Maybe you can try to import pdf's of the theatre. You can actually import a pdf from the import model menu. This pdf ends up on a separate layer in Capture, which you can then rename, apply a filter on etc.
The only issue is scaling, as this is not done very easily.But if your pdf source has a proper reference, you can get very decent results.
As Floriaan writes, this is already possible. The only thing you cannot do is place an actual image as an underlay, it would need to be vector based.