I'd start by figuring out how much gear to bring, then how to carry it.
The big question is how often are you willing and able to stop for food and water.
Let's assume you will be riding for 7 hours. You'll want to consume 250 calories and 500 - 750 ml of water (or energy drink) per hour. Plus, you should bring an extra tube, patch kit, a pump (and/or co2), cell phone, ID or insurance info and a multi-tool.
If you have 2 water bottles and stop every 2 hours, then you only need to carry 2-3 energy bars or snacks with you. Should be fine with a saddle bag and jersey pockets, or a small handlebar bag.
If you think you will need a jacket, I'd consider using a rack bag and carry an extra water bottle as well.
Personally I wouldn't put too much weight onto a rack that attaches to carbon seatstays; they normally aren't designed for those types of stresses. If you are talking about 10-15 lbs once a month, though, I don't think it's a problem.