This post is a "things that should be easily googleable but aren't" type post.
When an application is returning a dbus array of integers (Dbus type 'ai'), qdbusviewer will give you a message similar to:
unable to find method <MethodName> on path <DbusPath> in interface <Interface>
This message is sign of a failing in qdbusviewer and not in the application (as I discovered to today after a couple of hours of confusion!)
The solution is to use another dbus client, for example:
$ dbus-send --session --type=method_call --print-reply --dest=<Interface> <DbusPath> <MethodName> method return sender=:1.165 -> dest=:1.171 reply_serial=2 array [ int32 0 int32 1 ]
Hopefully that will save someone some pain.