I can definitely understand wanting larger bottles. I'd probably at least try upgrading to 16oz bottles, too, if I could find some.
You might take a look at the waistbelt systems with real water bottles. Most of them either have the bottles too close together for a backpack, or they put a pouch in between the bottles, making it uncomfortable with a backpack. However, you can probably slice off one of those pouches in between the bottles.
This might be a good option:
But you'd have to spend some time looking at them in person to know which one you could slice a pouch off of. Another problem might be side-to-side jiggling once the pouch is cut off - the pouch may help keep the bottles from moving. If you could cleanly strip off the pouch, you could even add a strong velcro attachment to add and remove it - I even thought about doing this with my backpack, to attach it to the hydration belt.
If the waistbelt is naturally small enough, or you have a pretty big waistline, this one would work well:
The middle section looks like you could rig up longer loops pretty easily to spread the distance between the bottles. That, coupled with some trimming of the foam (so it didn't dig into your butt cheeks), would probably work pretty well.
Another idea I've played with is having a small backpack, coupled with a real waistpack. Keep the essentials in the waistpack, and your overnight gear and food in the small backpack. This way, you can quickly ditch your small backpack (in a safe place) to summit a peak from a saddle/pass in the middle of your route. This idea is what got me thinking about hydration belts in the first place, especially since most of my backpacking trips are really just a means to summit peaks and do long traverses across ridgelines in Colorado's ranges. The other idea is to use the velcro attachment idea I had above, to able to quickly add/remove the pouch from your backpack/belt.
Overall, the 21oz capacity of the Amphipod system works pretty well for me, and it ensures I have to intentionally go into my backpack to drink the last bit of my water. This works well with a tablet/aquamira system, where you need to wait a period of time before drinking - a psychologically troublesome issue to do when you're sitting there at the water source, have no water, and are thirsty. It also works well for reminding me to stay properly fueled with food. Appetites can be difficult at altitude. For these reasons, stopping every 21oz seems to work well with my hiking flow.
Keep in mind, with a 2L platypus in the pack, you can certainly carry more water, when necessary. It's just on your back, instead of on your waistbelt. My baseweight is low enough that a 3 day trip is about 11 lbs of packweight, including 12oz of water in the platypus, so the extra waterweight, and not having a waistbelt for the pack is generally no big deal. 15 lbs of packweight is about the limit for my shoulders for long days. 11 lbs is super comfortable - not really noticable. I have pretty weak shoulder muscles though.
Above 15 lbs packweight, I will go straight to a heavy framed 2lb+ pack - usually because I'm carrying technical gear or a lot of water for the desert, which means a LOT more than 15 lbs.