Since macOS no longer displays a title for the NSOpenPanel perhaps you’d consider changing
[panel setTitle: juceStringToNS (owner.title)];
[panel setMessage: juceStringToNS (owner.title)];
That would display the prompt above the NSOpenPanel (not the prettiest option… but it does work).
You could make this only for 10.11 and later where the title bar no longer exists.