macOS NSSavePanel subclass

[MrFusion]

Did anyone ever managed to get NSSavePanel to accept folders as a valid choice? Googling this question, I can tell it has been asked ad nauseam. However, I didn't find a working solution. It should be possible, since NSOpenPanel is a subclass of NSSavePanel. In my case, both a folder and a filename are valid export destinations.

It's easy to set an empty extension and use the "filename" as a folder to be created, but this doesn't allow the user to pick an existing folder.
Ignoring the filename is also possible, but is not so nice from a GUI perspective.

If you select a folder, but leave the filename empty, the return button is grayed out. How do I gain control over this button?

I tried

- (BOOL)panel:(id)sender isValidFilename:(NSString *)filename

but this only gets called when the user hits return.

I tried subclassing, but

-(BOOL) validateUserInterfaceItem:(id <NSValidatedUserInterfaceItem>)anItem

is never called.

Any other suggestions?