[arjie]

Mozilla's Persona seems like an option. You can self-host and it seems to do what you want.

[GrinningFool]

Thanks, I hadn't heard about that - it's very close to what I'm looking for. I'll be taking a closer look - though without uptake across major service providers, it'll likely remain niche.

(I say this after having only skimmed it - could be wrong.)