That would be useful. I am filing a feature request for it.
Another approach would be to have an extension-specific property to set files executable on file creation, this would behave more like umask, but be restricted to specific file types. I don't know if it would be a blanket executable attribute, or have separate options for owner, group, other. Or we could have a File > Set Attributes... command which would make it easy to change read/write/executable status of the current file after the fact.