A Carradice seatbag will easily carry everything you need. Even the smaller size will do. Same for one of Rivendell's "Baggins" bags (made by Frost River). But you will need a saddle with saddlebag loops or some adapters for one. See www.wallbike.com
for Carradice bags. Any backpack will also do fine. A large camelbak or other hydration pack with some significant storage may be ideal if you want to really go light.
For what you are doing, the only clothes you need take are two sets of cycling clothes and a pair of nylon shorts to slip over your cycling shorts. When you arrive at each day's destination, you switch clothes and wash out the dirty set. If you like, you can carry additional clotes depending on capacity of your bag or pack.
You will also need the standard flat repair/multitool stuff that you should always carry. You should carry rain gear and a small toiletries kit. Buy travel sized soap, toothpaste, etc. to keep the weight and volume low. Some Woolite repackaged into a hotel size shampoo bottle should get you through four or five days washing your clothes. Same for sunscreen.
If you take simple solid color jerseys you won't look so conspicuous when you're off the bike. If you use MTB shoes you don't need to carry separate off bike shoes.
My guess is that you should be able to get the weight down to under 3 lbs if you try. If you need to do any camping, the weight shoots up significantly, but can still be done with a large saddlebag.