Good question - I have not tried to create campaigns from the API before so I don't have any specific experience to share about that.
I'm pretty sure the Eloqua API is totally open, you should be able to do whatever it can do. Last I checked, there are parts of the REST API that are not documented - that is, you can find options not in the documentation there by playing around with the endpoints to see what they do - but I believe pretty much the entire application is exposed via the API.
