Just like solar battery charging, it's not worth the effort in most situations.
I experimented with solar cooking at home. Some of my tests were done in the summer in 90 degree+ temps. Unless you're going to purchase and carry specialized gadgets and you have the luxury of placing them in a stationary position oriented to the sun just right for much of the day you're not going to get very good results as far as actual cooking goes.
I was thinking we could possibly even add water to a ziploc bag and wear it on the outside of our pack while hiking to hydrate up some foods
And when you rest, what are you going to seek? Shade. If you're going to carry that extra water to heat your food why don't you just carry a far lighter amount of extra fuel to do the same job much better, without the hassle and the risk of leaks?
or, utilize aluminum foil or the space blankets as reflective materials to use at camp in the afternoon in order to cook with.
Have you actually done this? It's not as easy as it sounds. You'll need some sort of struts or frame. What if it's windy? Again, a little extra fuel is much lighter and simpler.
When you get to camp you can use what you already have e.g. Platys or water bottles. Just leave them out in the sun and they'll be warm soon enough.
Solar showers do heat water quite well under the specific conditions I listed above. But if you take one along for the sole purpose of heating water to save fuel, once again the fuel itself will be much lighter and simpler.
If you really want to save fuel, plan some no cook meals. When I kayak camp on lake Mojave during August in 100 degree+ temps the last thing I want in the evening is a hot meal anyway. You'll see, the heat and exertion will sap your appetite.
What you propose may make sense under ideal conditions on a very long trip with no resupply but not for the relatively short jaunt you have planned.