Is there a planned ADS-B update that would allow ATC to view an aircraft's traffic coverage?
No, this is not being planned as a future update. The current ADS-B mandates of USA and Europe foresee ADS-B version 2 (EUROCAE ED-102A / RTCA DO-260B for 1090ES) to be implemented in 2020. Work on the development of version 3 of the standard has started but the retransmission of received data is not foreseen currently. Because of 1090 MHz spectrum overload, integrity, security and latency issues I don't think it is a particular good idea either. 

For oceanic areas ADS-B coverage will be provided by satellites equipped with ADS-B receivers. Aireon is a company that will use the next generation of Iridium satellites to provide oceanic ADS-B coverage for ADS-B.