Backpack, Masterwork: This backpack has numerous pockets for storing various items that might be needed while adventuring. Hooks are included for attaching items such as canteens, pouches, or even a rolled-up blanket. It has padded bands that strap across the chest and the waist to distribute its weight more evenly... When wearing a masterwork backpack, treat your Strength score as +1 higher than normal when calculating your carrying capacity.
Yes, I searched this issue; there are discussions about wearing both, they don't answer the following:
In Pathfinder Society play can a Bag of Holding/Handy Haversack be "upgraded" to have "Masterwork Backpack (linked, you have to scroll)" as their base form (i.e. pay the extra 50g, your bag of holding/haversack will have "Hooks ... for attaching items ... padded bands that strap across the chest and the waist to distribute its weight more evenly..." and (importantly) "When wearing a masterwork backpack, treat your Strength score as +1 higher than normal when calculating your carrying capacity.")
It seems to me that this should be the case, as there is no reason that a masterwork backpack cannot be used for crafting a bag of holding/haversack instead of a "regular" backpack.
(At the very least, the Haversack, which has multiple pockets like the Masterwork Backpack would seem to be subject to this upgrade).
I can't speak for Society play, but logically no, this would not work. The description of the "Masterwork Backpack" (a ludicrous item to start, if you ask me) specifically mentions multiple extra straps for the waist and torso, much like a modern hiker's backpack. A haversack, on the other hand, is more similar to a messenger bag or a large purse, while a bag of holding is simply a bag. The straps are arguably the most important part of the masterwork backpack, and make little to no sense for these items. You could argue for a "Masterwork Backpack of Holding," but then you are getting into custom item territory.
In Pathfinder Society play can a Bag of Holding/Handy Haversack be "upgraded" to have "Masterwork Backpack (linked, you have to scroll)" as their base form (i.e. pay the extra 50g, your bag of holding/haversack will have "Hooks ... for attaching items ... padded bands that strap across the chest and the waist to distribute its weight more evenly..." and (importantly) "When wearing a masterwork backpack, treat your Strength score as +1 higher than normal when calculating your carrying capacity.")
It seems to me that this should be the case, as there is no reason that a masterwork backpack cannot be used for crafting a bag of holding/haversack instead of a "regular" backpack.
(At the very least, the Haversack, which has multiple pockets like the Masterwork Backpack would seem to be subject to this upgrade).
Well, with a Bag of Holding it "appears to be a common cloth sack about 2 feet by 4 feet in size" so I believe changing that to a masterwork backpack would probably (correct me if I'm wrong) violate the PFS rules on alterations to the form and materials that an item is made from...
The Handy Haversack description might be more workable... but as the entire bag only weighs 5lbs at that point I don't think a really encumbrance obsessed GM would let you save much there.
With an actual Bag of Holding your best bet is probably to store the actual Bag of Holding IN a masterwork backpack. You've then dodged the rules for modifying magic items completely and still get everything you are looking for.
Thanks for the advice =p. That works great for small characters with low str... -- The Masterwork BP weighs 4 lbs for medium characters (1 lb for smalls)... and encumbrance changes in 3 and 4 lb increments below 10 STR. So for medium chars, it's either no gain or a 1 lb loss!