I've been trying to fit CalDAV/CardDAV to that use case for some time, but given how the majority of any kind of useful implementation of that is in PHP (which itself is fine, it's just not my forte), or Python (Apple's server implementation), I've all but given up, setting sights on the big players you mention. But maybe there is something here. The hangup will be acting as a provider to the devices you care about - Apple has APIs in to contacts and calendars, but I wonder if they'll be as useful as native accounts would be? A fun thought experiment for a bit...