Would curios support for the backpack be a possibility in the future? Just have it equip to the back slot in curios instead of the chest slot.

This is against the purpose of the backpack

Sorry to drop in out of nowhere but I do want to mention I was originally also very much against backpacks from BetterStorage / Wearable Backpacks being used in a way I did not design them to. Namely, they took up the chestplate slot, and had to be placed down to be accessed. I thought this was to improve immersion and offer an interesting choice to players.

However, not everyone plays the kinds of packs I do in the same way I do. They might not be interested in my style of backpacks if I don't make them more approachable. So I eventually changed my mind, because it enabled more players and modpack creators to configure them in the way they wanted to.

Both your and my backpacks will still be more balanced than other mods' in-inventory expansion solutions even if they don't take up the chestplate slot. With other mods adding other items to be worn in such a Curios slot (say, an Elytra, with the Curious Elytra mod) there is still a choice to be made.

(It is understandable if you'd not like to have a dependency on Curios for this one feature.)
I came across this as I was working on my own modpack today.

Namely, they took up the chestplate slot, and had to be placed down to be accessed. I thought this was to improve immersion and offer an interesting choice to players.

I think the one argument I don't understand is the fact that they need to take up the chestplate slot. I can kinda understand the elytra side of things as it is wings and it is hard to attach wings to your back and still put on a chestplate that wasn't designed for that. From a realism standpoint though, there should be no problem putting a backpack on while also wearing armor. I mean it literally goes around your arms. I can understand having to take it off to look at the contents inside, but I don't understand why you shouldn't be able to wear both armor and it at the same time.

Though I do understand Vazkii's point in not really wanting/needing to support this compat as there are other backpack mods that already have said features. While I still think it would be nice, it isn't really 100% needed.

@SaltSouls It introduces a choice to the player: Do you want more armor, or more inventory? The choice itself is similar to what you already have with the Elytra in Vanilla.