This is worth a look:
http://bikepackersmagazine.com/2015-faces-of-the-tour-divide/A lot run bladders in their frame bags (that is what I am planning). A bottle under the down tube is also common as a backup. (It will get covered in muck of course - just pour into your clean drinking bottle).
I met up with a 20 day racer on the weekend who had only 3 litres capacity (hydration bladder in his handlebar bag) and found this OK. Some take a Sea to Summit silnylon backpack the size of a tennis ball for lugging extra water/food for those days where they are concerned. Guzzle down a lot of water at each town and make sure you know where the wells are in the Basin. Wamsutter to Savery seems to have been the toughest water section in 2015 before hitting NM.
King Cage make a bottle mount which goes on top of the stem which is cool if your cockpit has space. Some run bottles in their feedbags.
The clear consensus is that if you can ride without a backpack, do so. Weight off the back and also off your bottom. Even some with hydration bags were likely using them as secondary water storage (e.g. Josh Kato) with the primary storage on the bike somewhere. I'm 5'5" so I will likely need a backpack, however it won't have water in it except on maybe the first 3 hours in a 2 litre Platypus bottle out of town on a couple of dry days (Basin / NM). My fork does not have cage mounts on it, but some people clamp them on with hose clamps or get them bonded onto carbon forks. I'm aiming to have 5.5 litres capacity - 2.5 in the bladder and 3.0 in Platypus bottles in the feedbag and backpack on the dry sections.
If you go with bottles, check how tightly they stay in and perhaps sort out a strapping system so they don't get ejected on day one. No-one wants to be "that guy".
What sort of capacity have other people found to be appropriate? I've heard some at 7 litres, some at 3 litres - 4kg is a big weight difference.
Thanx KiwiDave! I checked out that like and it looks like most are keeping the H2O on the bike somewhere and not on their backs. Are you saying that you will average 5.5L a day?