No, NFS, but that's not relevant to the problem. The filesystem contains media files, added to by Plex and also a user specifically for that. To keep it all running smoothly, everything needs to be group writable and although I can manually adjust existing files, anything that gets added is wrong and I have to manually adjust. Ok when it's me adding the content, but when Plex is recording, I don't know when it's taking place and trying to ensure correct perms gets harder and harder. So I just want to ensure anything that gets created is created with the required perms. That would make my life a lot simpler.
However, as I mentioned, these are XFS filesystems and as far as I can determine, XFS does not support a umask mount option, so I'll have to do it by user instead. Not absolutely ideal, but acceptable and more important, doable.
One is actually a mergerfs merge and I added a umask option to that mount as it was supposed to work (according to trapexit), but it gets it all wrong and applies the same perms to both files and folders, including all existing ones and then will not allow any changes. So I've reversed that.
As I said, setting umask for the appropriate users will do.