Indeed, iOS is going to be a challenge for the reasons you mentioned. The fact that we won't be able to have a standalone app like Awala on Android[1], which serves local apps, means we won't be able to have a single app that people could use to sync data with couriers.
So, we're going to have to pick a poison. Either we don't offer the sneakernet on iOS, or we'll leave it up to the developer of every app to use an SDK to sync with couriers (meaning that the user would have to open each app to sync with couriers, one app at a time). "Fortunately", Android is pretty much the only mobile OS in the regions susceptible to Internet blackouts, so the former option is the most likely one.
I think I've seen an app like the one you describe, but I can't remember the name. There's also Bridgefy, which is more general purpose, but they provide an SDK that you may be able to integrate. I haven't checked Bridgefy in ages, but I hope they sorted out their security; it didn't use any cryptography for ages, and when they added it was all quite vague and opaque.
[1] https://play.google.com/store/apps/details?id=tech.relaycorp...
Unfortunately at least on iOS, it's always going to be severely limited by what Apple allows and does not allow apps to do in the background and with regards to P2P connecting to other (especially non-Apple) Wi-Fi devices.
I've been dreaming about an optionally P2P messenger for a long time, i.e. something that just communicates over the internet while it's there, but lets messages piggy back onto others' devices if not.
This could address simple cases like one person with and another without a mobile data plan, or something more complicated like a hiker passing through a campsite, collecting everybody's pending outgoing messages and submitting them once they have signal again. (The return path in the latter scenario would be much more complicated, as it would require either a lot of storage and broadcasting, or good heuristics about who's going to pass by where, in order to preemptively download messages.)