Hi Karl (and to all who might be interested in this topic),
at this point of time, we are not selling banner ad space to third parties.
In the future, there may be an option made available to book a spot in the rotation, but right now it is not possible, I am afraid.
Do note that you could add a small e.g. 480x68, or 720x100 static (i.e. not animated) clickable banner to your signature. This would be displayed free of charge once per thread page to all users who have not disabled signature images.
Sorry to disappoint. If things change in the future, we'll let people know.