unix:dir= addresses are exactly the same as unix:tmpdir= addresses,
already supported by GDBus, except they forbid use of abstract sockets.
This is convenient for situations where abstract sockets are
impermissible, such as when a D-Bus client inside a network namespace
needs to connect to a server running in a different network namespace.
An abstract socket cannot be shared between two processes in different
network namespaces.

Applications could use unix:path= addresses instead, so this is only a
convenience, but there's no good reason not to support unix:dir=.
Currently it is not supported simply because unix:dir= is a relatively
recent addition to the D-Bus spec.