> Lately I've been pondering how one might create a "probably a human"/skin-in-the-game system.

This has the same energy as the "we need benchmarks for LLMs" startups. Like sure it's obvious and you can imagine really complex cathedrals about it. But nobody wants that. They "just" want Apple and Google to provide access to the same APIs their apps and backends use, associating authentic phone activity with user accounts. You already get most of the way there by supporting iCloud login, which should illuminate to you what you are really asking for is to play outside of Apple's ecosystem, a totally different ask.