Added some space to my packs -
A small duffel (9x21) lashed to the top or bottom adds some extra space. Kind of like a small sleeping bag. Gives me room for a bit more on my day and mid-sized packs.

Good call! You can keep stuff together that way, too.

Great idea.

I'm looking to add a water bottle pocket to a camera pack. Going to try to do it by mounting one of these with large snaps at each corner.

I've been using that device for years to give me backup carrying capacity, especially on my small EDC back pack. I had a shoe repair shop sew two small Fastex lash patches to the bottom of my Tom Binh Synapse daypack. As needed, I can fit to those patches a nylon belt pouch, ~9" x 6" x 2", that BQ used to sell. The pouch rides in the pack, filled with sundries. This arrangement can give me about an extra 100+ cu. in. of storage. Enough, for example, to fill & carry my plastic water bags, if I have to walk home. The patches also have other cargo carry options.

Its mostly for light things - fleece, blankets, shoes, food, etc.

Strapping stuff to the outside of the backpack is the typical solution when you need to carry a bit more than might fit inside. Good backpack should provide lashing options readily available (e.g. side compression cord is the most versatile). The rule of thumb - heavy stuff rides closer to the top, light things (like a jacket) - to the bottom. That helps to maintain balance while moving in the woods.

Versatility is enhanced when you can strap stuff to the outsdie of the pack, but danglies make it easy to get hung up on brush, rocks, etc. The ideal is a fully loaded, smoothly contoured pack that slips effortlessly past obstacles (I can dream, can't I??)

Isn't MOLLE good only with compatible modules?

Nah, you can attach just about anything to it. Snap link, zip ties, string, they all work to attach things.