Most manufacturers have male/female styles to their equipment lines. You will have to judge for yourself on how large a bag you will need but given the Nahanni area (rugged REMOTE wilderness) AND the need to bring all your food with you (no resupply available) I'd think you would require an expedition size pack. That's about 70-100 litres in size and capable of 50-80 lbs. There are people who have done remote trips with smaller packs but they've trimmed their gear over years of experience.

I would suggest logging onto the formun sections at backpacker.com and backpackinglight.com for more advice. But you will have to be prepared to be more specific on your needs and requirements before anyone can give you directed advice.

Advice:
http://www.adventurenetwork.com/cgi-bin/adventurenetwork/Backpack_Best_Worst_Features.html http://www.patc.net/hiking/gear/packs.html

A couple of examples below:
http://www.gregorypacks.com/prod.php?ID=6
http://www.ospreypacks.com/Packs/CrescentSeriesMens/Crescent85/

The type of pack design you suggest doesn't sound like it would be structurally sound for carrying larger loads. But there are dozens of packs out there that will give you side zipper access, bottom compartment access, removable top compartments, and strap on exterior pockets. A large amount of velcro on the exterior to attach other items is just asking for trouble - from simply attracting plants, pollen, dirt to getting yourself hung up on bushes, etc.
Most packs are also size adjustable for different back lengths and waist sizes.