Further, there's no good literature on what one's supposed to do in this case, or how to architect one's organization to be resilient to such situations.

Perhaps not specifically tailored for open source software projects, but areas such as key person risk and business continuity aren't exactly under-researched. The "trick" is to know you need it, and it's not a particularly pleasant conversation.

GitHub would actually be a good home for something like this. A secure repository (as secure as cloud-hosted can be) of keys and stuff, and a mechanism for "opening the vault" and naming new administrators (say, a unanimous vote of n out of m listed contributors).