This reminds me that the registration on the accessibility bus is currently done by calling the Embed() method of the org.a11y.atspi.Socket interface on the AT-SPI registry object itself which is, quite frankly, horrific.
There’s really no need to register an application to begin with: just asking for all connected clients if they expose the root AT-SPI interface is enough. There’s literally no reason to have a separate registry daemon.