Contents of external items are not visible to AT-SPI2
In AT-SPI2, external panel items show up as a "panel" element containing a single empty "filler" element.
This can be verified by examining the panel in Accerciser.
This lack of accessibility information limits the kind of alternative interfaces to the panel that can be created by an external program.
(See https://github.com/madewokherd/xalia for a description of my particular use case.)