Even when I've considered using Ghostwalk with Spelljammer, I've thought that it would be easier to expand the influence of the True Afterlife to an entire crystal sphere, rather than try to have some sort of bubble around Manifest, with the True Afterlife applying to people who die near Manifest and something else, like the Planescape outer planes, applying to people who die a long way from Manifest.
However, there is not too much to the True Afterlife. And the one-way gate makes it difficult to imagine any of the cool planewalking stuff that happens in the Great Wheel Cosmology happening in the True Afterlife. (Unless there was some sort of "Second Veil of Souls" on the other side of the True Afterlife that led to the Outer Planes.)
Most of what is said about the True Afterlife is in the "Beyond the Veil" section of Chapter 3 (on pages 121-134). The section suggests that a visit to the True Afterlife and back would change Ghostwalk forever (and should be the epic end of a Ghostwalk campaign). (And I've read elsewhere, that this was cut material, that was reinserted to make up the size of the book.)
There is a mention of the Ethereal Plane only connecting to the True Afterlife via the Veil of Souls, but I don't see any mentions of other planes. From that, I kind of infer that the people of Ghostwalk have no ability to ever connect to the Outer Planes, except when they pass through the Veil of Souls, and then they always go to the True Afterlife and can never return (unless brought back from the dead).
Over in the Classic Play: Book of the Planes cosmology vs Planescape topic, Ripvanwormer said that Purgatorium, from Classic Play: Book of the Planes might be a good fit for the True Afterlife:
Has anyone else thought about how to use the True Afterlife?ripvanwormer wrote:The main reason I'd use a realm like Purgatorium is to give the Pathfinder RPG's psychopomps somewhere to call home. That game has an extensive set of creatures whose role is to guide and catalogue souls, and it'd be good if they had neutral ground where they could set up their courts and libraries. It's also a more detailed afterlife that might be interesting in conjunction with Ghostwalk.