> "Don't tell anyone"
Just BP with cavers. Due to volume limits in tight passages, all caving beverage containers are birectional. You use them to haul liquids in and you use them to haul liquids out. Then triple rinse.
A lot of energy drink aluminum cans / aluminum bottles have 1-1/8" ID openings. Depending on the subject of the last dream, that can be a snug fit or a no-go. Rockstar comes in 24 ounce cans (although that a very thin aluminum - hardly more than soda can size).
Venom and many others are 16.9 ounce and ThinQ is 12 ounces and those are quite substanial. Okay, maybe not as UL as possible, but I wouldn't fret about their strength when multi-tasking as pee bottles, water bottles and cook pots.
From my own research, an adult male bladder holds, at most, 16 ounces. YMMV.
All those aluminum bottles I listed come with a screw top. While they offer big savings if triple-purposed, I'd bring an extra screwtop. It would just be too easy to drop off a mountain, etc.
I regret not setting aside more Avian bottles before they went to narrow-mouth lids. That's one reason I snag these earlier-version, wide-mouth, thicker-walled aluminum bottles from the recycling center. I can imagine them being "optimized" over time and then we'll be fighting over the old ones like we already do with Heinekien cans.
I'll do the deed in the sleeping bag, but when kneeling or on all fours. Better to go with gravity than perpendicular to it.