Hi Rob
Sorry I didn't get the picture in the first part very clearly.
It won't let you have both the segment add contacts hourly until campaign is deactivated, and the allow contacts to re-enter the campaign box checked in settings at the same time, or you will get the error.
I think it should be allowed, but it's currently not. This is just a work around.
When a contact is going to leave your campaign, instead send them to a decision rule, that is the same as your segment feeder, and have the evaluation time period as long as you can. If the contact ever meets the criteria to follow the yes path of that decision rule, you can send them back up your campaign canvas to any step. This will mimic re-entry into the campaign flow.
http://topliners.eloqua.com/docs/DOC-3216
If this answer has helped you then mark my answer as a correct answer.
Thanks
Faizal Reza