With an external base station that talks a different variety of RF, I think you could get an IoT system to "stacks on a coin cell".

Heck, Electric Imp is close to that size. https://electricimp.com/

I don't think you'd put a full system in a cable, but you'd make smart cables like Apple's lightning cable or their video adapters that actually convert signals.

See arduboy, covered previously: https://news.ycombinator.com/item?id=7325486

http://bateske.com/arduboy/

Thin film batteries + solar cells are already being mass manufactured so thats not a problem. RF chips are shrinking rapidly too and you'd need little more than a CC3000 from TI which is already pretty small (16.3 x 13.5 x 2mm)

Can't you power it briefly via a large enough capacitor/antenna pair?