I've actually done this in our platform, we use an object called "general" for this exact purpose. Obviously, you can name it whatever you want, but it's entirely possible (and maybe even recommendable).
In our case, we use as simple a form of relations as possible, so we have for example the namespace 'users', which has an object called 'general', which has either relation 'write' or relation 'read'. We still use separate subjects for each permission tuple though, however, this is not neccessary, you can use a generic name for the subject as well - though I'm note sure that's as recommendable 😊