As far as I have been able to find, there are no standard hydration bladders that fit neatly behind the s40 seat. I've run a TerraCycle pack with a 3L Camelback bladder attached to the headrest frame and with the two bottom attachment points fixed to the lower water bottle cage mounting holes on the seat. Works fine, but I've always wanted something tucked in a bit better. Although it seems like there should be more, there is surprisingly little room back there when you actually measure things. I just finished a prototype pack that fits tight against the back of the seat and holds two Platypus Hoser 1L hydration bladders (one on each side). It looks very much like a tiny pair of pants.
It mounts to the bottom cage mounts on each side of the seat and to the two holes at the top of the seat (don't know why they're there, but they were convenient and I didn't have to drill them!) There are three Velcro tabs on each side that pull the pack tight to the edge of the seat (with reflective tape 'cus, why not?) and a connection at the bottom to hold the two "legs" together under the point where they split around the seat stays. The bladder "pockets" have a 1" bellows pleat built in so they can open up to fit the bladder in, compressions straps to hold it all tight against the back of the seat and adjustable 1/8" bungee cord to even out compression across the width of the pack (and holding random stuff). The bit where the compression strap is attached at the bottom also make a pocket to hold the bite valve. (I'm going to say that was a purposeful design decision rather than happy accident - that's my story and I'm sticking to it!)
Even when full, the pack doesn't extend beyond a line from the outside rim of the seat across the top of the frame rail. I might make a few more modifications after testing this on the bike for a while, but this is pretty close to what was in my head.
Can't upload pics at the moment, but I've posted pics on my Facebook page here: /ross.jantz.35/posts/2386271991603350
(Ok, the forum won't let me post a full URL, either. So just copy, paste into the address bar and add the prerequisite "facebook" stuff to the beginning)