Yeah, those things were designed decades before the invention of bladders, and I rarely used one with mine. (given, I live in a much cooler climate) I'm sure you could modify it with minimal cutting and sewing. On other packs I've had luck with a bit of paracord and a small "key-ring" type carabiner to hang it and just ran the hose out under the lid.
However, you may want to look at another older school option. Given the limited capacity of the bag you have, it makes sense to mount a canteen pouch on the waist belt instead of tying up volume. Might take some rigging, but I like one tucked right back at the frame out of the way. As a bonus, I feel less like a gerbil when I can drink that way.
And for an even simpler option, I have used Vapur bottles quite a bit the last few years. They have a built in clip, and I just attach them to the bottom of the shoulder strap, and they hang flat and relatively out of the way.