It is difficult to find a good built in water holder. Even on my Osprey daypack it's not all that easy to reach back to get the bottle out and even harder to get it back in.

I've tried different water bottle holders in the past, including the velcro type you mention. When it's new it stays in place most of the time but with age it will fall off. A closed loop makes sense if I can find a good spot for it.

I like your idea of going with a 1/2 liter bottle. If I need more i could keep some in a light weight collapsable bottle in my pack but like you said it would have less pull on my waste belt.

It's good to see that someone else has been experimenting.

My big effort will be getting down to 30 lbs on my backpack. To me it's a balance between weight and comfort, meaning having a comfortable sleeping bag, pad, etc. Sleeping good at night is just as important as carrying less weight. I think 30 lbs would be a good balance.