Thanks Pete, perhaps I'll check in with Opus.
In Opus, you open the main window, then click the "Shortcuts" button and then you can assign a joystick button to some common functions.
I'm not sure how Opus reads the button press; whether it 'intercepts' P3D or reads directly. I'll ask them, but I suspect it intercepts because if I disable controllers in P3D the buttons don't work, and if I re-enable then they do, so some how they are related.
I wondered about a mouse macro since the update weather is available from an add-on menu directly in P3D, but on re-reading that section last night I understood it related to gauges and not general mouse ops. Never mind; though I have found something else I can use a mouse macro for, so that was handy!
If I find an answer from Opus, I'll post here.
Cheers.