There are many lives that are good without access to private land. There are many aspects of permaculture that are not gardening-based. There are even ways to access land that are not ownership based.
Community garden plots - even better, organizing and helping in community gardens. Community gardens don't only grow vegetables, they grow outreach. People engage with the people they see working, and learn what is happening and what is possible.
Care for people - volunteering in libraries, schools, school gardens, senior homes, your own extended family or neighborhood. For some people, even politics. Working on local water quality or land use issues. Learning how to work collectively for goals that are larger than personal.
Gardening on waste land - the parking strip, vacant lots. We have a couple of parks that used to be garbage dumps and are pretty barren. I occasionally go put local acorns and fruit seeds in the ground there. Do any come up? Who knows. Do the ground squirrels eat them all? Maybe. I still find it worthwile. Despite our teminology, nothing is permanent. A tree that lives for five years rather than fifty is still a net benefit.
Education. Both in schools and elsewhere. I'm a docent at the local native plant garden. Lead nature hikes and foraging outings in the city and in parks. The more people know about our natural world, the more they care about the Earth.
Production. We don't have to grow the produce to participate in the production of our sustenance. Brewing, fermentation, fiber arts, cooking, woodworking, natural building, repairs of all sorts, clean transportation; as you can see by the forums these are all part of permaculture and all are important to how we live well.