Kbarnett, one of the reasons I originally replied to your post was that I am planning that same trip in mid Sept around the 17th to the 20th.
I am going to drive to Newport Beach, park my car in a secure location and pedal towards San Diego. I am doing it on my hardtail MTB with panniers and sleeping bag.
I am camping the first night at Doheny State Beach at their Hike & Bike campsite which is only $6 per night. And then camping at Campland on The Bay at Mission Bay in San Diego the second night. Camping is $35 there but it has full amenities such as showers, laundry, pool, jacuzzi, grill,store etc.
I am looking forward to it. It should be a great tour.
I was thinking why not just order some cheaper panniers online and bring them with you when you fly to the US, instead of trying to rent some. Some moderate size ones flattened down shouldn't take up too much size in your suitcase. I don't know about AUS but you can get a set here in the US online for around $35+/- if you shop a bit.
Just a thought.