Unfortunately that is not as easy as it sounds.
The build in 2d dwg/dxf export is not exposed to API. …as I mentioned in this devenloper topic.
The solution - by DanRathbun - that can be found there is perhaps not the most “expected” solution because it need user interaction, but it can be good enough. However, it does not fully cover the request here, since it is exporting the views predefined in a plugin, and not the scenes.
There is also an alternative in this topic (I’m not sure how suitable it is…)