How would this integrate to Diaspora? In TRAVELLER you would have a refuelling spot which did not obviously appear on star-charts, perfect for pirates or simply those who want to have an intermediate hop to populated stars. On the other hand, solar-cells do not work, everything has to be powered by fuelled fusion reactors, and they are scooping hydrogen from the gas-giant more often and it is a massive one, more gravity, more risky if the scoop orbits go bad.
In DIASPORA a culture either gets along with a space-station entirely powered by fusion, or a tiny planet that orbits the gas-giant somewhere. If the culture declined but their fusion engines keep going, with a large reserve of fuel in standby mode, they have limited sources of light but everything outside is inky starry blackness, so they probably lost interest in their stellar neighbours. There was a good movie called City of Ember, based on a juvenile novel, where a city was built underground to weather a nuclear war, and 500 years after everyone hid, a box is supposed to pop open with instructions on how to leave the city and get back to the surface -- but the box was lost. So the people are trapped with little idea of their past, they have abandoned technology that no longer works, and their main generator is failing (the whole underground settlement gets dark and they must launch flares occasionally to let people move around and function until the generator is working again). This might be a good basis for an adventure dealing with outsiders.