You're flying ETOPS over the Ocean
Is it possible to get basic weather data via ground support? [...]with some development effort a data feed could be made into a viable backup.
...like, building a network of weather radars in the middle of the North atlantic? msbbarratt, that's not going to happen.
What would you do?
try asking for nearby aircraft who could provide some guidance? phone home and ask meteorologists for the best and most current data and advice they have? and as Meikleour said, deviate extensively.