I just linked a bunch of organizations that are supporting green roofs to this group. Hopefully over time we can reach out to them to get their expertise.

The questions which we'll need help answering are:

how much weight is put on a 'green roof' - that is how much weight must the existing roof structure be able to support? Are there ways to reinforce existing roofs?

what plants are good for SF weather patterns? Ideally we'd need robust enough plants that people wouldn't have to water them all the time or even much at all. Water is already a concern in CA without starting to water numerous gardens. (Also, can you imagine lugging enough water up to a roof? perhaps using rain barrels?)