For my use-case, I ended up having to switch to Azure DevOps, which is extremely awkward with GitHub, but at least has free Windows/Linux/MacOS support, and can at least be bothered responding to customers.
It was extremely new when I posted, so I didn't know about it.
I've no idea why AppVeyor has gotten so bad with dealing with customers, but this should obviously be handled better.
Sorry guys for the lack of response here. First of all, we are looking to support macOS in on-premise AppVeyor Server offering.
Regarding hosted environment - it's still unclear what virtualization technology could be used: VMWare, Anka or something else. At the moment we are operating Windows and Linux images in 4 different clouds. Introducing Mac cloud is not a trivial task though we don't see much interest from the customers/community in supporting Mac builds on AppVeyor :-/
Alternatively, would it work for you if you are able to connect your own Mac VM to AppVeyor and run builds there?