I don't use a hydration bladder anymore, but I do fill up a bladder for in-camp use. Where I camp, it can drop below freezing pretty much any night, so I had the idea of stuffing the bladder into my pillow sack, both to keep the water from freezing and to give my pillow some weight so it's less likely to slip away. I might put the water filter in there also if I can keep it positioned to one end so it doesn't jab me in the face.

I know it's a risk, having water close to the precious down quilt, but the pillow sack is effectively a dry bag, so that risk seems minimal. What I'm really trying to mitigate is leaking inside the pillow because I usually have extra clothes and stuff sacks in the pillow to bulk it up because I am a side-sleeper.

Has anyone tried this? Am I asking for trouble? Or is there a simple way to mitigate leakage so I can sleep without worry?

For many years, I just used an MSR 2-liter Dromedary or Dromlite bag as my pillow. If it had water in it, it had a bit of a waterbed bounce to it, but it worked fine and I never had a leak. If it didn't have water in it, I would use the smallest valve and blow it up like a football - worked as well or better than any of the air pillows they sell today. (Nowadays, I keep my raingear in my pack's lid compartment. The compartment clips to my pack, so it's easy to clip it off at night, use that as the pillow, and clip it back on in the morning. That assumes I don't have a down or pile jacket to use.)

It will work, but you are taking a calculated risk. Try it on an overnight trip in pleasant warm weather (where failure isn't critical) and see what happens.

Any water container filled with a couple of liters of water won't even come close to freezing overnight if the temperatures are simply 'below freezing'. The very worst you'd experience would be a very light skin of ice on the periphery of the bladder and that would probably require something closer to 20 degrees overnight. Your water filter is better off sealed in a plastic bag and put inside the sleeping bag with you on nights you are worried about sub-freezing temps.

I can't say how safe or comfortable a pillow it would make. I just use some clothes items inside a stuff sack.

Yeah, truly the water filter is the most important thing to protect. I usually have it in a baggie, in my pocket. However, it's not unusual for it to drop to the 20's or below overnight, even in summer. I probably shouldn't worry about the bladder except if it's close to depleted.

I'll still stuff extra clothes, stuff sacks, etc. in the pillow, but I've shifted to wearing more to sleep and bringing less extra clothing, so I'm looking to use the bulk and weight.

I'm glad I'm not the only one who's thought of this. Even so, I'll just try to be careful and try it on a warmer, low-risk night.

I'm the nervous nellie type (sorry Nellie!) so I wouldn't chance it. But if you think the bladder is robust enough, why not just put it at the bottom of your quilt/sleeping bag so you're not putting any pressure on it, but still keeping it warmer than outside temps?

I actually tried this. In my case, it didn't go well. Even though the bladder I was using had never leaked before, the weight of my head caused most of the water to leak out of the space between the opening and the cap during the night. As others have suggested, try this out first in a safe situation.

Hmmm. That worries me a bit, then. I've seen bladder tests where they run over them with trucks and such. I'd hope my noggin isn't heavier than a truck!

If things warm up a little here, maybe I'll string up a hammock in the yard and try it while napping.